Current sensor

APPLICATION NOTE—105
Application Note 105
December 2005
Current Sense Circuit Collection
Making Sense of Current
Tim Regan, Editor
INTRODUCTION
Sensing and/or controlling current flow is a fundamental This Application Note Will Change
requirement in many electronics systems, and the tech- This Application Note is a growing and changing docu-
niques to do so are as diverse as the applications them- ment. Many of the chapters listed below are placeholders
selves. This Application Note compiles solutions to cur- for material that will be filled in soon. As the chapters are
rent sensing problems and organizes the solutions by added, their links will be enabled.
general application type. These circuits have been culled
from a variety of Linear Technology documents. Using the Application Note
Click the name of a chapter in the “Circuit Collection In-
Circuits Organized by General Application dex” below to open the PDF version of that chapter.
Each chapter collects together applications that tend to
solve a similar general problem, such as high side cur- Contributors
rent sensing, or negative supply sensing. The chapters Jon Munson, Alexi Sevastopoulos,
are titled accordingly (see “Circuit Collection Index” be- Greg Zimmer, Michael Stokowski
low). In this way, the reader has access to many possible
solutions to a particular problem in one place. , LTC, LTM, LT, Burst Mode, OPTI-LOOP, Over-The-Top and PolyPhase are registered
trademarks of Linear Technology Corporation. Adaptive Power, C-Load, DirectSense, Easy
Drive, FilterCAD, Hot Swap, LinearView, µModule, Micropower SwitcherCAD, Multimode
It is unlikely that any particular circuit shown will exactly Dimming, No Latency ∆Σ, No Latency Delta-Sigma, No RSENSE, Operational Filter, PanelPro-
tect, PowerPath, PowerSOT, SmartStart, SoftSpan, Stage Shedding, SwitcherCAD, ThinSOT,
meet the requirements for a specific design, but the sug- UltraFast and VLDO are trademarks of Linear Technology Corporation. Other product names
gestion of many circuit techniques and devices should may be trademarks of the companies that manufacture the products.

prove useful. Specific circuits may appear in several
chapters if they have broad application.

CIRCUIT COLLECTION INDEX

Current Sense Basics Level Shifting High Speed
High Side High Voltage Fault Sensing
Low Side Low Voltage Digitizing
Negative Voltage High Current (100mA to Amps) Current Control
Unidirectional Low Current (Picoamps to Precision
Bidirectional Milliamps) Wide Range
AC Motors and Inductive Loads
DC Batteries

Introduction-1

APPLICATION NOTE 105: Current Sense Circuit Collection

Current Sense Basics
This chapter introduces the basic techniques used for HIGH SIDE CURRENT SENSING
sensing current. It serves also as a definition of common
Current sensed in the supply path of the power connec-
terms. Each technique has advantages and disadvan-
tion to the monitored load. Current generally flows in just
tages and these are described. The types of amplifiers
one direction (uni-directional). Any switching is per-
used to implement the circuits are provided.
formed on the load-side of monitor.
To see other chapters in this Application Note, return to
DC VSUPPLY
the Introduction.
+
LOW SIDE CURRENT SENSING RSENSE ISENSE OUTPUT ∝ ILOAD

–
Current sensed in the ground return path of the power
connection to the monitored load. Current generally ILOAD LOAD

flows in just one direction (uni-directional). Any switch-
ing is performed on the load-side of monitor.

DC VSUPPLY
High Side Advantages
Load is grounded
ILOAD LOAD VCC Load not activated by accidental short at power con-
+
nection
RSENSE ISENSE OUTPUT ∝ ILOAD
High load current caused by short is detected
–
High Side Disadvantages
. High input common mode voltages (often very high)
Output needs to be level shifted down to system oper-
ating voltage levels
Low Side Advantages
Low input common mode voltage Amplifier Types for High Side Implementation
Ground referenced output voltage Dedicated current sensing amplifiers: LT6100,
Easy single supply design LTC6101, LT1787
Over-the-Top™ op amps: LT1637
Low Side Disadvantages Flying capacitor amplifier: LTC6943
Load lifted from direct ground connection
Load activated by accidental short at ground end load
switch
High load current caused by short is not detected

Amplifier Types for Low Side Implementation
Precision zero-drift op amps: LTC2050, LTC2054
Instrumentation amplifiers: LTC2053, LT1990,
LTC6943
Rail-to-Rail Input op amps: LT1677

Current Sense Basics-1

FULL-RANGE (HIGH AND LOW SIDE) SUMMARY OF CURRENT SENSE SOLUTIONS
CURRENT SENSING
The next few pages contain a table that summarizes cur-
Bi-directional current sensed in a bridge driven load, or rent sense solutions and applicable devices. Look first in
unidirectional high side connection with a supply side the “Type/Circuit” column and the “Gain” column for a
switch. general description of the application. Then scan across
the other columns for applicable devices and their speci-
DC VSUPPLY
fications.

VCC

RSENSE
+
LOAD ISENSE OUTPUT ∝ ILOAD

ILOAD –

Full-Range Advantages
Only one current sense resistor needed for bidirec-
tional sensing
Convenient sensing of load current on/off profiles for
inductive loads

Full-Range Disadvantages
Wide input common mode voltage swings
Common mode rejection may limit high frequency
accuracy in PWM applications

Amplifier Types for Bi-directional Implementation
Difference amplifiers-LT1990, LT1991, LT1995,
LT1996
Instrumentation amplifiers: LTC2053
Flying capacitor amplifier: LTC6943



ACCURACY SPEED

OFFSET INPUT DIFFERENTIAL
GAIN DEVICES AND VSUPPLY
TYPE/CIRCUIT VOLTAGE CURRENT BANDWIDTH SLEW RATE VIN RANGE (VCM) VIN RANGE
(V/V) PACKAGES RANGE (VS)
(VOS) (IBIAS) (SURVIVAL)
High Side 10 to 50 LT6100 300µV 5µA 100kHz 0.05V/µs 2.7V to 36V (VS + 1.4V) to 48V ±48V
One Direction
Voltage Out MSOP-8
RSENSE
VIN
DFN
LOAD
(VCC + 1.4V) TO 48V
1 8
VS– VS+
RG1 RG2
5k 5k

– + R
25k
A1

VCC
2
2.7V TO 36V –
VOUT
Q1 RE A2 5
10k +
VO1

RO
R R/3
50k
VEE FIL A2 A4
4 3 6 7
6100 F01

High Side Resistor LTC6101 350µV 250nA 200kHz 2.5V/µs 4V to 70V (VS – 1.5V) to 70V ±70V
One Direction Ratio LTC6101HV 350µV 250nA 200kHz 2.5V/µs 4V to 105V (VS – 1.5V) to 105V ±105V
Current Out
ILOAD VSENSE
SOT23-5
– +
VBATTERY
RSENSE
RIN 5
MSOP-8
10V V+
L
O
A
D
IN – 5k –
3

IN + 5k +
4 IOUT
10V
OUT ROUT
1 VOUT = VSENSE x
RIN
LTC6101/LTC6101HV V–
2 ROUT
6101 BD


ACCURACY SPEED

High Side Fixed 8 LT1787 75µV 20µA 300kHz 0.1V/µs 2.5V to 36V 2.5V to 36V ±10V
Bi-directional or LT1787HV 75µV 20µA 300kHz 0.1V/µs 2.5V to 60V 2.5V to 60V ±10V
Current or Voltage (ROUT = 20k) Scaleable
RSENSE ISENSE SO-8
VS– VS+
MSOP-8
RG1A RG2A
1.25k 1.25k
FIL– FIL+
RG1B RG2B
1.25k 1.25k

– +
A1
IOUT

VBIAS
ROUT
Q1 Q2 20k
VOUT

VEE CURRENT MIRROR
1787 F 01

High Side Resistor LT1494 150µV 250pA 3kHz 0.001V/µs 2.1V to 36V 0 to VS + (36V – VS) 36V
One Direction Ratio LT1636 50µV 5nA 200kHz 0.07V/µs 2.6V to 44V 0 to VS + (44V – VS) 44V
Voltage Out LT1637 100µV 20nA 1MHz 0.35V/µs 1.8V to 44V 0 to VS + (44V – VS) 44V
Over the Top Amplifiers LT1672 150µV 250pA 12kHz 0.005V/µs 2.1V to 36V 0 to VS + (36V – VS) 36V
3V TO 44V LT1782 400µV 8nA 200kHz 0.07V/µs 2.2V to 18V 0 to VS + (18V – VS) 36V
R1
200Ω
LT1783 400µV 45nA 1.25MHz 0.42V/µs 2.2V to 18V 0 to VS + (18V – VS) 36V
3V
RS
+
LT1784 1500µV 250nA 2.5MHz 2.4V/µs 2V to 18V 0 to VS + (18V – VS) 36V
0.2Ω
Q1
LT1637
2N3904
– VOUT
ILOAD
R2
(0V TO 2.7V)
DIP-8
VOUT 2k
LOAD ILOAD =
(RS)(R2/R1) 1637 TA06 MS-8
SO-8
DFN
SOT23-5
SOT23–6


ACCURACY SPEED

High Side Resistor LTC2053 5µV 4nA 200kHz 0.2V/µs 2.7V to 11V 2.7V to 11V 5.5V
One Direction Ratio LTC6800 5µV 4nA 200kHz 0.2V/µs 2.7V to 5.5V 2.7V to 5.5V 5.5V
Voltage Out
Instrumentation Amplifier DFN
5V
0.1µF
MS-8
3
+ 8
7
LTC2053 VOUT
2 6
VIN – 5
4
1

0.1µF VOUT = –VIN

2053 TA07
–5V

High Side or Low Side Unity LTC6943 6pA 90kHz 5V to 18V 5V to 18V 18V
One Direction
Voltage on a TSSOP – 16
capacitor output
Flying Capacitor
E
I
POSITIVE OR
NEGATIVE RAIL RSHUNT

1/2 LTC6943
11 12

10

1µF 1µF E I= E
RSHUNT
9

6 7

14 15
0.01µF

6943 • TA01b


ACCURACY SPEED

High Side or Low Side 1 and 10 LT1990 900µV 2.5nA 105kHz 0.55V/µs 2.4V to 36V –250V to 250V ±250V
Bi-Directional 1 to 13 LT1991 15µV 110kHz 0.12V/µs 2.7V to 36V –60V to 60V ±60V
Voltage Out 1 to 7 LT1995 1000µV 2.5nA 32MHz 1000V/µs 5V to 36V 0V to 36V VS + 0.3V
Difference Amplifiers 9 to 117 LT1996 15µV 38kHz 0.12V/µs 2.7V to 36V –60V to 60V ±60V
V S+
8
9
10
M9
M3
7 Pin Strap SO-8
VIN – M1

VIN + 1
LT1991
6

R1
Configurable DFN
P1
R2* 2 5 10k
10k 3
P3
P9 4
MS–10
V + – VIN –
ILOAD = IN
V S– 10kΩ
*SHORT R2 FOR LOWEST OUTPUT
OFFSET CURRENT. INCLUDE R2 FOR
HIGHEST OUTPUT IMPEDANCE.

Low Side Resistor LTC2050 0.5µV 75pA 3MHz 2V/µs 2.7V to 7V 0V to (VS – 1.3V) VS + 0.3V
One Direction Ratio LTC2054 0.5µV 0.6pA 500kHz 0.5V/µs 2.7V to 7V 0V to (VS – 0.7V) VS + 0.3V
Voltage Out LTC2054HV 0.5µV 0.6pA 500kHz 0.5V/µs 2.7V to 12V 0V to (VS – 0.7V) VS + 0.3V
Zero-Drift Amplifiers
5V SO-8
3
+
5 OUT
3V/AMP
SOT23-5
1
4
LTC2050HV
LOAD CURRENT
IN MEASURED SOT23 – 6
– 2
CIRCUIT, REFERRED
TO –5V
10Ω 10k

TO 3mΩ
MEASURED
CIRCUIT
LOAD CURRENT 0.1µF
– 5V 2050 TA08

Low Side Resistor LT1218 25µV 30nA 300kHz 0.1V/µs 2V to 36V 0V to VS VS + 0.3V
One Direction Ratio LT1677 20µV 2nA 7.2MHz 2.5V/µs 2.5V to 44V 0V to VS VS + 0.3V
Voltage Out LT1800 75µV 25nA 80MHz 25V/µs 2V to 12.6V 0V to VS VS + 0.3V
Rail to Rail I/O Amplifiers LT1806 100µV 1µA 325MHz 125V/µs 1.8V to 12.6V 0V to VS VS + 0.3V
IL 3V
LT6200 1400µV 10µA 110MHz 50V/µs 2.2V to 12.6V 0V to VS VS + 0.3V
0A TO 1A
52.3Ω + LT6220 70µV 15nA 60MHz 20V/µs 2.2V to 12.6V 0V to VS VS + 0.3V
VOUT
LT1800
0V TO 2V
0.1Ω –

52.3Ω 1k SO-8
1800 F02 DIP-8
VOUT = 2 • IL
f–3dB = 4MHz
UNCERTAINTY DUE TO VOS, IB < 4mA
SOT23-5
SOT23 – 6



High Side
This chapter discusses solutions for high side current “Classic” Positive Supply Rail Current Sense
sensing. With these circuits the total current supplied to 5V
a load is monitored in the positive power supply line.
200Ω
the Introduction.
0.2Ω +
LT6100 Load Current Monitor LT1637
Q1
200Ω 2N3904
TO LOAD RSENSE
– 0V TO 4.3V
LOAD ILOAD 2k
C1
+
1 8 0.1µF 5V VOUT = (2Ω)(ILOAD) 1637 TA02

V S– V S+
2
VCC A4
7 This circuit uses generic devices to assemble a function
+
3V
C2 – + similar to an LTC6101. A Rail-to-Rail Input type op amp
0.1µF
3 6
is required since input voltages are right at the upper rail.
FIL A2
The circuit shown here is capable of monitoring up to
44V applications. Besides the complication of extra parts,
4
VEE
OUT 5
OUTPUT the VOS performance of op amps at the supply is gener-
LT6100 ally not factory trimmed, thus less accurate than other
6100 F04
solutions. The finite current gain of the bipolar transistor
This is the basic LT6100 circuit configuration. The inter- is a small source of gain error.
nal circuitry, including an output buffer, typically operates
from a low voltage supply, such as the 3V shown. The Over-The-Top Current Sense
monitored supply can range anywhere from VCC + 1.4V 3V TO 44V
up to 48V. The A2 and A4 pins can be strapped various R1
200Ω
ways to provide a wide range of internally fixed gains.
3V
The input leads become very hi-Z when VCC is powered
RS
down, so as not to drain batteries for example. Access to 0.2Ω +
Q1
an internal signal node (pin 3) provides an option to in- LT1637
2N3904
clude a filtering function with one added capacitor. Small- – VOUT
(0V TO 2.7V)
ILOAD
signal range is limited by VOL in single-supply operation. R2
VOUT 2k
LOAD ILOAD =
(RS)(R2/R1) 1637 TA06

This circuit is a variation on the “classic” high-side cir-
cuit, but takes advantage of Over-the-Top input capability
to separately supply the IC from a low-voltage rail. This
provides a measure of fault protection to downstream
circuitry by virtue of the limited output swing set by the
low-voltage supply. The disadvantage is VOS in the Over-
the-Top mode is generally inferior to other modes, thus
less accurate. The finite current gain of the bipolar tran-
sistor is a source of small gain error.

High Side-1

Self-Powered High Side Current Sense Precision High Side Power Supply Current Sense
1.5mΩ
VREGULATOR

2 – 8
OUT
7 100mV/A
LTC6800
OF LOAD
3 + 6 10k CURRENT
5
4
0.1µF
ILOAD LOAD

150Ω

6800 TA01

This is a low-voltage, ultra-high-precision monitor featur-
This circuit takes advantage of the microampere supply ing a Zero-Drift Instrumentation Amplifier (IA) that pro-
current and Rail-to-Rail input of the LT1494. The circuit vides Rail-to-Rail inputs and outputs. Voltage gain is set
is simple because the supply draw is essentially equal to by the feedback resistors. Accuracy of this circuit is set
the load current developed through RA. This supply cur- by the quality of resistors selected by the user, small-
rent is simply passed through RB to form an output volt- signal range is limited by VOL in single-supply operation.
age that is appropriately amplified. The voltage rating of this part restricts this solution to
applications of <5.5V. This IA is sampled, so the output is
High Side Current Sense and Fuse Monitor discontinuous with input changes, thus only suited to
RSENSE very low frequency measurements.
TO LOAD 2mΩ FUSE
BATTERY
BUS
+
Positive Supply Rail Current Sense
1 8
VS– VS+ VCC
ADC R1
2 7 200Ω
POWER VCC A4
≥2.7V C2 – +
0.1µF
Rs
3 6 0.2Ω – –
FIL A2 Q1
1/2 LT1366 1/2 LT1366
TP0610L
+ +
4
VEE
OUT 5 OUTPUT
2.5V = 25A
ILOAD

R2
VO = ILOAD • RS ( )
R2
R1
LT6100 LOAD 20k = ILOAD • 20Ω
DN374 F02
1366 TA01

The LT6100 can be used as a combination current sensor
and fuse monitor. This part includes on-chip output buff- This is a configuration similar to an LT6100 implemented
ering and was designed to operate with the low supply with generic components. A Rail-to-Rail or Over-the-Top
voltage (≥2.7V), typical of vehicle data acquisition sys- input op amp type is required (for the first section). The
tems, while the sense inputs monitor signals at the first section is a variation on the classic high-side where
higher battery bus potential. The LT6100 inputs are toler- the P-MOSFET provides an accurate output current into
ant of large input differentials, thus allowing the blown- R2 (compared to a BJT). The second section is a buffer
fuse operating condition (this would be detected by an to allow driving ADC ports, etc., and could be configured
output full-scale indication). The LT6100 can also be with gain if needed. As shown, this circuit can handle up
powered down while maintaining high impedance sense to 36V operation. Small-signal range is limited by VOL in
inputs, drawing less than 1µA max from the battery bus. single-supply operation.

High Side-2

Precision Current Sensing in Supply Rails Measuring bias current into an Avalanche Photo
E
Diode (APD) using an instrumentation amplifier.
I
POSITIVE OR
NEGATIVE RAIL 1k
RSHUNT 1%
VIN BIAS OUTPUT
1/2 LTC6943 10V TO 33V TO APD
35V
11 12
– CURRENT
LT1789 MONITOR OUTPUT
10
0mA TO 1mA = 0V TO 1V
1µF 1µF E I= E + A=1
RSHUNT
9 AN92 F02a

1N4684
1k
6 7 3.3V
1%
VIN BIAS OUTPUT
10V TO 35V TO APD

14 15 10M
0.01µF – CURRENT
6943 • TA01b
0mA TO 1mA = 0V TO 1V
+ A=1
This is the same sampling architecture as used in the
AN92 F02b
front-end of the LTC2053 and LTC6800, but sans op amp
gain stage. This particular switch can handle up to 18V, The upper circuit uses an instrumentation amplifier (IA)
so the ultra-high precision concept can be utilized at powered by a separate rail (>1V above VIN) to measure
higher voltages than the fully integrated ICs mentioned. across the 1kΩ current shunt. The lower figure is similar
This circuit simply commutates charge from the flying but derives its power supply from the APD bias line. The
sense capacitor to the ground-referenced output capaci- limitation of these circuits is the 35V maximum APD
tor so that under dc input conditions the single-ended voltage, whereas some APDs may require 90V or more.
output voltage is exactly the same as the differential In the single-supply configuration shown, there is also a
across the sense resistor. A high precision buffer ampli- dynamic range limitation due to VOL to consider. The ad-
fier would typically follow this circuit (such as an vantage of this approach is the high accuracy that is
LTC2054). The commutation rate is user-set by the ca- available in an IA.
pacitor connected to pin 14. For negative supply monitor-
ing, pin 15 would be tied to the negative rail rather than
ground.

High Side-3

Simple 500V Current Monitor Bidirectional Battery-Current Monitor
TO RSENSE
CHARGER/
LOAD C1
15V
1 8 1µF
–
FIL FIL+
–
LT1787
2 VS VS+ 7

3 VBIAS 6
DNC
ROUT
4 5
VEE OUTPUT
VOUT
C2 C3*
–5V 1µF 1000pF
1787 F02

*OPTIONAL

This circuit provides the capability of monitoring current
in either direction through the sense resistor. To allow
negative outputs to represent charging current, VEE is
connected to a small negative supply. In single-supply
Adding two external Mosfets to hold off the voltage al- operation (VEE at ground), the output range may be offset
lows the LTC6101 to connect to very high potentials and upwards by applying a positive reference level to VBIAS
monitor the current flow. The output current from the (1.25V for example). C3 may be used to form a filter in
LTC6101, which is proportional to the sensed input volt- conjunction with the output resistance (ROUT) of the part.
age, flows through M1 to create a ground referenced This solution offers excellent precision (very low VOS)
output voltage. and a fixed nominal gain of 8.

High Side-4

LTC6101 Supply Current Simple High Side Current
included as Load in Measurement Sense Using the LTC6101
V+ BATTERY BUS

RIN
RSENSE RIN
RSENSE 100Ω
4 3 0.01Ω
4 3
+ –
LOAD + –
LOAD 2 5
2 5

1 1 VOUT
LTC6101 VOUT LT6101
4.99V = 10A
ROUT
ROUT
4.99k
6101 F06 VOUT = ILOAD(RSENSE • ROUT/RIN) DN374 F01

This is a basic high side current monitor using the
This is the basic LTC6101 high-side sensing supply-
LTC6101. The selection of RIN and ROUT establishes the
monitor configuration, where the supply current drawn
desired gain of this circuit, powered directly from the
by the IC is included in the readout signal. This configu-
battery bus. The current output of the LTC6101 allows it
ration is useful when the IC current may not be negligible
to be located remotely to ROUT. Thus, the amplifier can
in terms of overall current draw, such as in low-power
be placed directly at the shunt, while ROUT is placed near
battery-powered applications. RSENSE should be selected
the monitoring electronics without ground drop errors.
to limit voltage-drop to <500mV for best linearity. If it is
This circuit has a fast 1µs response time that makes it
desirable not to include the IC current in the readout, as
ideal for providing MOSFET load switch protection. The
in load monitoring, pin 5 may be connected directly to V+
switch element may be the high side type connected be-
instead of the load. Gain accuracy of this circuit is limited
tween the sense resistor and the load, a low side type
only by the precision of the resistors selected by the user.
between the load and ground or an H-bridge. The circuit
is programmable to produce up to 1mA of full-scale out-
put current into ROUT, yet draws a mere 250µA supply
current when the load is off.

High Side-5

High-Side Transimpedance Amplifier Intelligent High Side Switch

The LT1910 is a dedicated high side MOSFET driver with
built in protection features. It provides the gate drive for
a power switch from standard logic voltage levels. It pro-
vides shorted load protection by monitoring the current
Current through a photodiode with a large reverse bias
flow to through the switch. Adding an LTC6101 to the
potential is converted to a ground referenced output volt-
same circuit, sharing the same current sense resistor,
age directly through an LTC6101. The supply rail can be
provides a linear voltage signal proportional to the load
as high as 70V. Gain of the I to V conversion, the trans-
current for additional intelligent control.
impedance, is set by the selection of resistor RL.

High Side-6

48V Supply Current Monitor with
Isolated Output and 105V Survivability

The HV version of the LTC6101 can operate with a total
supply voltage of 105V. Current flow in high supply volt-
age rails can be monitored directly or in an isolated fash-
ion as shown in this circuit. The gain of the circuit and
the level of output current from the LTC6101 depends on
the particular opto-isolator used.

High Side-7


Low Side
This chapter discusses solutions for low side current Precision Current Sensing in Supply Rails
sensing. With these circuits the current flowing in the E
I
ground return or negative power supply line is moni- POSITIVE OR
NEGATIVE RAIL
tored. RSHUNT

1/2 LTC6943
To see other chapters in this Application Note, return to 11 12

the Introduction.
10
“Classic” High-Precision Low Side Current Sense 1µF 1µF E I= E
RSHUNT
5V 9

3 5 OUT
+ 3V/AMP 6 7
1 LOAD CURRENT
LTC2050HV
4 IN MEASURED
– 2
CIRCUIT, REFERRED 14 15
TO –5V
0.01µF
10Ω 10k
6943 • TA01b
TO 3mΩ
MEASURED
CIRCUIT
LOAD CURRENT 0.1µF
This is the same sampling architecture as used in the
– 5V 2050 TA08 front-end of the LTC2053 and LTC6800, but sans op amp
gain stage. This particular switch can handle up to 18V,
This configuration is basically a standard non-inverting
so the ultra-high precision concept can be utilized at
amplifier. The op amp used must support common-mode
higher voltages than the fully integrated ICs mentioned.
operation at the lower rail and the use of a Zero-Drift type
This circuit simply commutates charge from the flying
(as shown) provides excellent precision. The output of
sense capacitor to the ground-referenced output capaci-
this circuit is referenced to the lower Kelvin contact,
tor so that under dc input conditions the single-ended
which could be ground in a single-supply application.
output voltage is exactly the same as the differential
Small-signal range is limited by VOL for single-supply
across the sense resistor. A high precision buffer ampli-
designs. Scaling accuracy is set by the quality of the
fier would typically follow this circuit (such as an
user-selected resistors.
LTC2054). The commutation rate is user-set by the ca-
pacitor connected to pin 14. For negative supply monitor-
ing, pin 15 would be tied to the negative rail rather than
ground.

Low Side-1

–48V Hot Swap Controller
GND
RIN
3× 1.8k IN SERIES
+ CL
1/4W EACH 100µF

CIN LOAD
GND 1 1µF R3
(SHORT PIN) 5.1k
R1 VIN EN
402k LTC4252-1 VOUT
1% *
8 2
OV PWRGD
9 7 RD 1M
UV DRAIN
R2 10 6 Q1
32.4k TIMER GATE
IRF530S
1% CT 3 4
SS VEE SENSE
0.33µF RC RS
C1 CSS 5 10Ω 0.02Ω
10nF 68nF CC
18nF

–48V
* M0C207

This load protecting circuit employs low-side current event of supply or load faults. An internal shunt regulator
sensing. The N-MOSFET is controlled to soft-start the establishes a local operating voltage.
load (current ramping) or to disconnect the load in the

–48V Low Side Precision Current Sense

The first stage amplifier is basically a complementary and furnishes a positive output voltage for increasing
form of the “classic” high-side current sense, designed load current. . A dual op amp cannot be used for this im-
to operate with telecom negative supply voltage. The plementation due to the different supply voltages for
Zener forms an inexpensive “floating” shunt-regulated each stage. This circuit is exceptionally precise due to the
supply for the first op amp. The N-MOSFET drain delivers use of Zero Drift op amps. The scaling accuracy is estab-
a metered current into the virtual ground of the second lished by the quality of the user-selected resistors. Small-
stage, configured as a trans-impedance amplifier (TIA). signal range is limited by VOL in single-supply operation
The second op amp is powered from a positive supply of the second stage.

Low Side-2

Fast Compact –48V Current Sense
VOUT = 3V – 0.1Ω • ISENSE
ISENSE = 0A TO 30A
ACCURACY ≈ 3%
VOUT
Q1 R1 1k
FMMT493 4.7k 1%
VS = 3V
30.1Ω
1% –
3.3k R1 REDUCES Q1 DISSIPATION
0805
LT1797
×3
+
0.1µF
SETTLES TO 1% IN 2µs,
BZX84C6V8 1V OUTPUT STEP
VZ = 6.8V 0.003Ω
1% 3W
–48V SUPPLY –48V LOAD
(–42V TO –56V) – + 1797 TA01

ISENSE

This amplifier configuration is essentially the comple- tance (1kΩ in this circuit). In this circuit, the output volt-
mentary implementation to the classic high-side configu- age is referenced to a positive potential and moves
ration. The op amp used must support common-mode downward when representing increasing –48V loading.
operation at its lower rail. A “floating” shunt-regulated Scaling accuracy is set by the quality of resistors used
local supply is provided by the Zener diode, and the tran- and the performance of the NPN transistor.
sistor provides metered current to an output load resis-

–48V Current Monitor

Low Side-3

In this circuit an economical ADC is used to acquire the and/or higher efficiency operation, the ADC may be pow-
sense resistor voltage drop directly. The converter is ered from a small transformer circuit as shown below.
powered from a “floating” high-accuracy shunt-regulated
supply and is configured to perform continuous conver-
sions. The ADC digital output drives an opto-isolator,
level-shifting the serial data stream to ground. For wider
supply voltage applications, the 13k biasing resistor may
be replaced with an active 4mA current source such as
shown to the right. For complete dielectric isolation

GND
RIN
3× 1.8k IN SERIES
+ CL
1/4W EACH 100µF

CIN LOAD
GND 1 1µF R3
(SHORT PIN) 5.1k
R1 VIN EN
402k LTC4252-1 VOUT
1% *
8 2
OV PWRGD
9 7 RD 1M
UV DRAIN
R2 10 6 Q1
32.4k TIMER GATE
IRF530S
1% CT 3 4
SS VEE SENSE
0.33µF RC RS
C1 CSS 5 10Ω 0.02Ω
10nF 68nF CC
18nF

–48V
* M0C207


Low Side-4

Simple Telecom Power Supply Fuse Monitor
47k
–48V 5V
RETURN FUSE
STATUS
R1 R2
100k 100k 3 MOC207
SUPPLY A SUPPLY B
RTN 47k VA VB STATUS STATUS
1 4 5V
VA OUT F OK OK 0 0
SUPPLY A OK UV OR OV 0 1
8 STATUS UV OR OV OK 1 0
VB UV OR OV UV OR OV 1 1
LTC1921 OK: WITHIN SPECIFICATION
2 MOC207
OV: OVERVOLTAGE
FUSE A
47k UV: UNDERVOLTAGE
5V
7 5 VFUSE A VFUSE B FUSE STATUS
FUSE B OUT A SUPPLY B
STATUS = VA = VB 0
= VA ≠ VB 1
≠ VA = VB 1
MOC207 ≠ VA ≠ VB 1*
6
OUT B 0: LED/PHOTODIODE ON
R3
47k 1: LED/PHOTODIODE OFF
F1 D1 *IF BOTH FUSES (F1 AND F2) ARE OPEN,
SUPPLY A 1/4W
–48V OUT ALL STATUS OUTPUTS WILL BE HIGH
–48V SINCE R3 WILL NOT BE POWERED
F2 D2
SUPPLY B = LOGIC COMMON
–48V

The LTC1921 provides an all-in-one telecom fuse and status flags are generated that indicate the condition of
supply-voltage monitoring function. Three opto-isolated the supplies and the fuses.

Low Side-5


Negative Voltage
This chapter discusses solutions for negative voltage To see other chapters in this Application Note, return to
current sensing. the Introduction.

Telecom Supply Current Monitor
+ 5V

LOAD IL 48V

– 3
+
7
G2
5 6
RS
LT1990 VOUT
2 8
– 4
G1
1
–77V ≤ VCM ≤ 8V REF
VOUT = VREF – (10 • IL • RS) VREF = 4V
4 5
IN OUT 174k 1nF
LT6650 1
GND FB
2
20k
1990 AI01

1µF

The LT1990 is a wide common-mode range difference mately 4V by the LT6650. The output signal moves
amplifier used here to amplify the sense resistor drop by downward from the reference potential in this connection
10. To provide the desired input range when using a sin- so that a large output swing can be accommodated.
gle 5V supply, the reference potential is set to approxi-

GND
RIN
3× 1.8k IN SERIES
+ CL
1/4W EACH 100µF

CIN LOAD
GND 1 1µF R3
(SHORT PIN) 5.1k
R1 VIN EN
402k LTC4252-1 VOUT
1% *
8 2
OV PWRGD
9 7 RD 1M
UV DRAIN
R2 10 6 Q1
32.4k TIMER GATE
IRF530S
1% CT 3 4
SS VEE SENSE
0.33µF RC RS
C1 CSS 5 10Ω 0.02Ω
10nF 68nF CC
18nF

–48V
* M0C207


Negative Voltage-1

–48V Low Side Precision Current Sense

The first stage amplifier is basically a complementary and furnishes a positive output voltage for increasing
form of the “classic” high-side current sense, designed load current. . A dual op amp cannot be used for this im-
to operate with telecom negative supply voltage. The plementation due to the different supply voltages for
Zener forms an inexpensive “floating” shunt-regulated each stage. This circuit is exceptionally precise due to the
supply for the first op amp. The N-MOSFET drain delivers use of Zero Drift op amps. The scaling accuracy is estab-
a metered current into the virtual ground of the second lished by the quality of the user-selected resistors. Small-
stage, configured as a trans-impedance amplifier (TIA). signal range is limited by VOL in single-supply operation
The second op amp is powered from a positive supply of the second stage.

ISENSE = 0A TO 30A
ACCURACY ≈ 3%
VOUT
Q1 R1 1k
FMMT493 4.7k 1%
VS = 3V
30.1Ω
1% –
0805
LT1797
×3
+
0.1µF
VZ = 6.8V 0.003Ω
1% 3W
(–42V TO –56V) – + 1797 TA01

ISENSE


Negative Voltage-2

–48V Current Monitor

In this circuit an economical ADC is used to acquire the and/or higher efficiency operation, the ADC may be pow-
sense resistor voltage drop directly. The converter is ered from a small transformer circuit as shown below.
powered from a “floating” high-accuracy shunt-regulated
supply and is configured to perform continuous conver-
sions. The ADC digital output drives an opto-isolator,
level-shifting the serial data stream to ground. For wider
supply voltage applications, the 13k biasing resistor may
be replaced with an active 4mA current source such as
shown to the right. For complete dielectric isolation

47k
–48V 5V
RETURN FUSE
STATUS
R1 R2
100k 100k 3 MOC207
SUPPLY A SUPPLY B
1 4 5V
VA OUT F OK OK 0 0
2 MOC207
OV: OVERVOLTAGE
FUSE A
5V
STATUS = VA = VB 0
= VA ≠ VB 1
≠ VA = VB 1
MOC207 ≠ VA ≠ VB 1*
6
R3
SUPPLY A 1/4W
F2 D2
–48V


Negative Voltage-3


Unidirectional
Unidirectional current sensing monitors the current flow- Unidirectional Current Sensing Mode
ing only in one direction through a sense resistor. RSENSE
TO
LOAD
2.5V TO
To see other chapters in this Application Note, return to C
0.1µF 60V
the Introduction.
1 8
Unidirectional Output into FIL– FIL+
LT1787HV +
A/D with Fixed Supply at VS+ 2 VS
–
VS 7

3 VBIAS 6
RSENSE DNC

5V ROUT
C1 4 5
5V
1 8 1µF VEE VOUT
FIL– FIL+ VOUT
–
LT1787
2 VS VS+ 7 R1 1787 F08
20k
3 VBIAS 6 IOUT 5%
DNC
ROUT
4 5 VCC
VEE +IN CS
VOUT LTC1286 CLK TO µP
–IN D
R2 VREF GND OUT
5k 1787 F06
5%

Here the LT1787 is operating with the LTC1286 A/D con-
verter. The –IN pin of the A/D converter is biased at 1V by
the resistor divider R1 and R2. This voltage increases as
sense current increases, with the amplified sense voltage
appearing between the A/D converters –IN and +IN ter- This is just about the simplest connection in which the
minals. The LTC1286 converter uses sequential sampling LT1787 may be used. The VBIAS pin is connected to
of its –IN and +IN inputs. Accuracy is degraded if the ground, and the VOUT pin swings positive with increasing
inputs move between sampling intervals. A filter capaci- sense current. The output can swing as low as 30mV.
tor from FIL+ to FIL– as well as a filter capacitor from Accuracy is sacrificed at small output levels, but this is
VBIAS to VOUT may be necessary if the sensed current not a limitation in protection circuit applications or where
sensed currents do not vary greatly. Increased low level
changes more than 1LSB within a conversion cycle.
accuracy can be obtained by level shifting VBIAS above
ground. The level shifting may be done with resistor di-
viders, voltage references or a simple diode. Accuracy is
ensured if the output signal is sensed differentially be-
tween VBIAS and VOUT.

Unidirectional-1

16-Bit Resolution Unidirectional 48V Supply Current Monitor with
Output into LTC2433 ADC Isolated Output and 105V Survivability

The LTC2433-1 can accurately digitize signal with source
impedances up to 5kΩ. This LTC6101 current sense cir-
cuit uses a 4.99kΩ output resistance to meet this re-
quirement, thus no additional buffering is necessary.

Intelligent High Side Switch


12-Bit Resolution Unidirectional
Output into LTC1286 ADC
RSENSE
TO I = 100A 0.0016Ω
LOAD
1 8 2.5V TO 60V
FIL– FIL+
LT1787HV +
The LT1910 is a dedicated high side MOSFET driver with 2 VS
–
VS 7
R1 C1
5V
built in protection features. It provides the gate drive for 3
DNC
VBIAS 6 15k 1µF

a power switch from standard logic voltage levels. It pro- ROUT
20k VREF VCC
4 5 CS
vides shorted load protection by monitoring the current VEE
VOUT
+IN
LTC1286 CLK TO µP
–IN
flow to through the switch. Adding an LTC6101 to the C2
D
GND OUT

same circuit, sharing the same current sense resistor, VOUT = VBIAS + (8 • ILOAD • RSENSE) 0.1µF LT1634-1.25
1787 TA01

While the LT1787 is able to provide a bidirectional out-
put, in this application the economical LTC1286 is used
to digitize a unidirectional measurement. The LT1787 has
a nominal gain of eight, providing a 1.25V full-scale out-
put at approximately 100A of load current.

Unidirectional-2


Bidirectional
Bidirectional current sensing monitors current flow in Practical H-Bridge Current Monitor Offers Fault
both directions through a sense resistor. Detection and Bidirectional Load Information

To see other chapters in this Application Note, return to –
BATTERY BUS DIFF
OUTPUT
the Introduction. TO ADC
+
LTC6101 RIN RIN LTC6101
Bidirectional Current Sensing ROUT ROUT
RS RS
with Single Ended Output
+
VS
FOR IM RANGE = ±100A,
DIFF OUT = ±2.5V
RS = 1mΩ
B A B A RIN = 200Ω
LOAD ROUT = 4.99k

IM

RS
100Ω 0.1 100Ω

I 100Ω DN374 F04
100Ω

4 3 5 5 3 4 This circuit implements a differential load measurement
for an ADC using twin unidirectional sense measure-
ments. Each LTC6101 performs high side sensing that
– –
LTC6101 LTC6101
rapidly responds to fault conditions, including load
+ +
shorts and MOSFET failures. Hardware local to the switch
2 1 1 2
module (not shown in the diagram) can provide the pro-
tection logic and furnish a status flag to the control sys-
2.5V
REF
5V tem. The two LTC6101 outputs taken differentially pro-
2.5k
+ duce a bidirectional load measurement for the control
LT1490 VOUT servo. The ground-referenced signals are compatible
2.5V TO 5V (CONNECTION A) – with most ∆ΣADCs. The ∆ΣADC circuit also provides a
2.5V TO 0V (CONNECTION B) “free” integration function that removes PWM content
0A TO 1A IN EITHER DIRECTION
2.5k
from the measurement. This scheme also eliminates the
need for analog-to-digital conversions at the rate needed
Two LTC6101’s are used to monitor the current in a load to support switch protection, thus reducing cost and
in either direction. Using a separate rail-to-rail op amp to complexity.
combine the two outputs provides a single ended output.
With zero current flowing the output sits at the reference
potential, one-half the supply voltage for maximum out-
put swing or 2.5V as shown. With power supplied to the
load through connection A the output will move positive
between 2.5V and Vcc. With connection B the output
moves down between 2.5V and 0V.

Bidirectional-1

Conventional H-Bridge Current Monitor Single Supply 2.5V Bidirectional Operation with
BATTERY BUS
External Voltage Reference and I/V Converter
+ ISENSE
TO RSENSE
CHARGER/
LOAD C1 2.5V + VSENSE(MAX)
1 8 1µF
FIL– FIL+
–
LT1787
RS + 2 VS VS+ 7
DIFF 2.5V
3 VBIAS 6
AMP DNC
IM – ROUT
C3
4 5 1000pF
VEE
VOUT
–
A1 VOUT A
2.5V + LT1495
1M
DN374 F03
5% LT1389-1.25
1787 F07

Many of the newer electric drive functions, such as steer-
ing assist, are bidirectional in nature. These functions are
generally driven by H-bridge MOSFET arrays using pulse- The LT1787’s output is buffered by an LT1495 rail-to-rail
width-modulation (PWM) methods to vary the com- op-amp configured as an I/V converter. This configura-
manded torque. In these systems, there are two main tion is ideal for monitoring very low voltage supplies. The
purposes for current monitoring. One is to monitor the LT1787’s VOUT pin is held equal to the reference voltage
current in the load, to track its performance against the appearing at the op amp’s non-inverting input. This al-
desired command (i.e., closed-loop servo law), and an- lows one to monitor supply voltages as low as 2.5V. The
other is for fault detection and protection features. op-amp’s output may swing from ground to its positive
supply voltage. The low impedance output of the op amp
A common monitoring approach in these systems is to may drive following circuitry more effectively than the
amplify the voltage on a “flying” sense resistor, as high output impedance of the LT1787. The I/V converter
shown. Unfortunately, several potentially hazardous fault configuration also works well with split supply voltages.
scenarios go undetected, such as a simple short to
ground at a motor terminal. Another complication is the Battery Current Monitor
noise introduced by the PWM activity. While the PWM IL
RSENSE
CHARGE
noise may be filtered for purposes of the servo law, in- 0.1Ω

formation useful for protection becomes obscured. The
DISCHARGE 5V 12V
best solution is to simply provide two circuits that indi- RA RA
vidually protect each half-bridge and report the bidirec-
–
–
A2 A1
tional load current. In some cases, a smart MOSFET 1/2 LT1495 RA RA 1/2 LT1495
+
+
bridge driver may already include sense resistors and
offer the protection features needed. In these situations, 2N3904 2N3904
the best solution is the one that derives the load informa-
tion with the least additional circuitry.
DISCHARGE
OUT
CHARGE
OUT
VO = IL ()
RB
RA
RSENSE

RB RB FOR RA = 1k, RB = 10k
VO
= 1V/A
IL 1495 TA05

One LT1495 dual op-amp package can be used to estab-
lish separate charge and discharge current monitoring
outputs. The LT1495 features Over-the-Top operation
allowing the battery potential to be as high as 36V with
only a 5V amplifier supply voltage.

Bidirectional-2

Fast Current Sense with Alarm The LT1995 is shown as a simple unity gain difference
amplifier. When biased with split supplies the input cur-
rent can flow in either direction providing an output volt-
age of 100mV per Amp from the voltage across the
100mΩ sense resistor. With 32MHz of bandwidth and
1000V/usec slew rate the response of this sense ampli-
fier is fast. Adding a simple comparator with a built in
reference voltage circuit such as the LT6700-3 can be
used to generate an over-current flag. With the 400mV
reference the flag occurs at 4A.

Bidirectional Current Sense with Separate Charge/Discharge Output
IDISCHARGE RSENSE ICHARGE
CHARGER

RIN D RIN C
100 100
RIN D RIN C
100 100
4 3 3 4
VBATT
L 2 + – 5 5 – + 2
O
A
D
1 1
LTC6101 LTC6101
+ +
ROUT D VOUT D VOUT C ROUT C
4.99k 4.99k
– –
6101 TA02

DISCHARGING: VOUT D = IDISCHARGE • RSENSE ( ROUT D
RIN D )
WHEN IDISCHARGE ≥ 0

CHARGING: VOUT C = ICHARGE • RSENSE ( ROUT C
RIN C )
WHEN ICHARGE ≥ 0

In this circuit the outputs are enabled by the direction of while the other LT6101, VOUT C, ramps from low to high
current flow. The battery current when either charging or in proportion to the charging current. The active output
discharging enables only one of the outputs. For example reverses when the charger is removed and the battery
when charging, the VOUT D signal goes low since the discharges into the load.
output MOSFET of that LTC6101 turns completely off

Bidirectional-3

Bidirectional Absolute Value Current Sense
IDISCHARGE RSENSE ICHARGE
CHARGER

RIN D RIN C

RIN D RIN C

4 3 3 4
VBATT
L 2 + – 5 5 – + 2
O
A
D
1 1
LTC6101 LTC6101
+
VOUT ROUT
–
6101 TA05

DISCHARGING: VOUT = IDISCHARGE • RSENSE ( ROUT
RIN D )
WHEN IDISCHARGE ≥ 0

CHARGING: VOUT = ICHARGE • RSENSE ( ROUT
RIN C )
WHEN ICHARGE ≥ 0

The high impedance current source outputs of two value of the magnitude of the current into or out of the
LTC6101’s can be directly tied together. In this circuit the battery. The direction or polarity of the current flow is not
voltage at VOUT continuously represents the absolute discriminated.

Full-Bridge Load Current Monitor
+VSOURCE 5V
LT1990
900k 10k 8
7

1M 100k
2
–
RS 6
VOUT
– + 3 1M
+
IL VREF = 1.5V
4
10k 5
IN OUT 54.9k 1nF
LT6650 40k 900k
GND FB 40k 100k

20k
–12V ≤ VCM ≤ 73V
VOUT = VREF ± (10 • IL • RS) 1 1990 TA01

1µF

The LT1990 is a difference amplifier that features a very the output away from ground. The output will move
wide common mode input voltage range that can far ex- above or below 1.5V as a function of which direction the
ceed its own supply voltage. This is an advantage to re- current in the load is flowing. As shown, the amplifier
ject transient voltages when used to monitor the current provides a gain of 10 to the voltage developed across
in a full bridge driven inductive load such as a motor. The resistor RS.
LT6650 provides a voltage reference of 1.5V to bias up

Bidirectional-4

Low Power, Bidirectional 60V Precision Hi Side Current Sense

Using a very precise zero-drift amplifier as a pre-amp the 60V limit of the LT1787HV circuit. Overall gain of this
allows for the use of a very small sense resistor in a high circuit is 1000. A 1mA change in current in either direc-
voltage supply line. A floating power supply regulates the tion through the 10mΩ sense resistor will produce a
voltage across the pre-amplifier on any voltage rail up to 10mV change in the output voltage.

Split or Single Supply Operation, Bidirectional Output into A/D
1Ω
1%
IS = ±125mA VCC
5V
VSRCE 1 8
FIL– FIL+
≈4.75V LT1787
–
2 VS VS+ 7 10µF
16V
3 VBIAS 6 1
DNC
7
20k CONV
VEE 4 5 VOUT (±1V) 2 6 CLOCKING
VEE AIN LTC1404 CLK
–5V VOUT 3 CIRCUITRY
OPTIONAL SINGLE VREF 5
DOUT
SUPPLY OPERATION: 10µF GND
DISCONNECT VBIAS 16V 4 8
FROM GROUND
AND CONNECT IT TO VREF. 10µF DOUT
REPLACE –5V SUPPLY 16V
WITH GROUND. VEE 1787 TA02
OUTPUT CODE FOR ZERO –5V
CURRENT WILL BE ~2430

In this circuit, split supply operation is used on both the LT1787 pin 6 is driven by VREF, the bidirectional meas-
LT1787 and LT1404 to provide a symmetric bidirectional urement range is slightly asymmetric due to VREF being
measurement. In the single-supply case, where the somewhat greater than mid-span of the ADC input range.

Bidirectional-5


AC
Sensing current in ac power lines is quite tricky in the
sense that both the current and voltage are continuously
changing polarity. Transformer coupling of signals to
drive ground referenced circuitry is often a good ap-
proach.

the Introduction.

Single Supply RMS Current Measurement

The LT1966 is a true RMS-to-DC converter that takes a
single-ended or differential input signal with rail-to-rail
range. The output of a pcb mounted current sense trans-
former can be connected directly to the converter. Up to
75A of AC current is measurable without breaking the
signal path from a power source to a load. The accurate
operating range of the circuit is determined by the selec-
tion of the transformer termination resistor. All of the
math is built in to the LTC1966 to provide a dc output
voltage that is proportional to the true rms value of the
current. This is valuable in determining the power/energy
consumption of ac powered appliances.

AC-1


DC
DC current sensing is for measuring current flow that is The power introduced to the elements, and thereby their
changing at a very slow rate. temperature, is ascertained from the voltage-current
product with the LT6100 measuring the current and the
To see other chapters in this Application Note, return to LT1991 measuring the voltage. The LT6100 senses the
the Introduction. current by measuring the voltage across the
10Ω resistor, applies a gain of 50, and provides a ground
Micro-Hotplate Voltage and Current Monitor referenced output. The I to V gain is therefore
VDR+ 500mV/mA, which makes sense given the 10mA full
scale heater current and the 5V output swing of the
10Ω
1%
LT6100. The LT1991’s task is the opposite, applying pre-
VS– VS+
IHOTPLATE
cision attenuation instead of gain. The full scale voltage
+ –
of the heater is a total of 40V (±20), beyond which the life
5V VCC CURRENT
MONITOR
of the heater may be reduced in some atmospheres. The
LT6100
VEE A2 A4
VOUT = 500mV/mA
LT1991 is set up for an attenuation factor of 10, so that
MICRO-HOTPLATE
BOSTON
the 40V full scale differential drive becomes 4V ground
MICROSYSTEMS
MHP100S-005 referenced at the LT1991 output. In both cases, the volt-
5V
5V
M9
ages are easily read by 0V–5V PC I/O cards and the sys-
M3
M1
LT1991
VOLTAGE
MONITOR
tem readily software controlled.
P1 V + – VDR–
P3 VOUT = DR
10
P9
Battery Current Monitor
VDR– 6100 TA06

www.bostonmicrosystems.com IL
RSENSE
CHARGE
0.1Ω
Materials science research examines the properties and
interactions of materials at various temperatures. Some DISCHARGE 5V 12V
of the more interesting properties can be excited with – RA RA
–
localized nano-technology heaters and detected using the A2 A1
1/2 LT1495 1/2 LT1495
presence of interactive thin films. + RA RA
+

While the exact methods of detection are highly complex 2N3904 2N3904

and relatively proprietary, the method of creating local- DISCHARGE
OUT
CHARGE
OUT
VO = IL ()
RB
RA
RSENSE

ized heat is as old as the light bulb. Shown is the sche- RB RB FOR RA = 1k, RB = 10k

matic of the heater elements of a Micro-hotplate from VO
IL
= 1V/A
1495 TA05

Boston Microsystems (www.bostonmicrosystems.com).
The physical dimensions of the elements are tens of mi- One LT1495 dual op-amp package can be used to estab-
crons. They are micromachined out of SiC and heated lish separate charge and discharge current monitoring
with simple DC electrical power, being able to reach outputs. The LT1495 features Over-the-Top operation
1000°C without damage. allowing the battery potential to be as high as 36V with

DC-1

Bidirectional Battery-Current Monitor High Side Current Sense and Fuse Monitor
TO RSENSE RSENSE
TO LOAD 2mΩ FUSE
CHARGER/ BATTERY
LOAD C1 BUS
15V
1 8 1µF 1 8 +
FIL– FIL+
LT1787 VS– VS+
–
2 VS V S+ 7 ADC 2 7
POWER VCC A4
3 VBIAS 6 ≥2.7V
DNC C2 – +
0.1µF
ROUT
4 5 3 6
VEE OUTPUT FIL A2
VOUT
C2 C3*
–5V 1µF 1000pF
4 OUT 5 OUTPUT
1787 F02 VEE 2.5V = 25A
LT6100
*OPTIONAL
DN374 F02

This circuit provides the capability of monitoring current The LT6100 can be used as a combination current sensor
in either direction through the sense resistor. To allow and fuse monitor. This part includes on-chip output buff-
negative outputs to represent charging current, VEE is ering and was designed to operate with the low supply
connected to a small negative supply. In single-supply voltage (≥2.7V), typical of vehicle data acquisition sys-
operation (VEE at ground), the output range may be offset tems, while the sense inputs monitor signals at the
upwards by applying a positive reference level to VBIAS higher battery bus potential. The LT6100 inputs are toler-
(1.25V for example). C3 may be used to form a filter in ant of large input differentials, thus allowing the blown-
conjunction with the output resistance (ROUT) of the part. fuse operating condition (this would be detected by an
This solution offers excellent precision (very low VOS) output full-scale indication). The LT6100 can also be
and a fixed nominal gain of 8. powered down while maintaining high impedance sense
inputs, drawing less than 1µA max from the battery bus.
“Classic” Positive Supply Rail Current Sense
5V Gain of 50 Current Sense
ISENSE RSENSE
200Ω VSUPPLY
6.4V TO 48V
+
LT6100 VS VS– LOAD
0.2Ω +
Q1
LT1637 + –
200Ω 2N3904
– 0V TO 4.3V
5V VCC
LOAD ILOAD 2k

VOUT = (2Ω)(ILOAD) 1637 TA02 FIL
VOUT
50 • RSENSE • ISENSE
This circuit uses generic devices to assemble a function VEE A2 A4
6100 TA04

similar to an LTC6101. A Rail-to-Rail Input type op amp
is required since input voltages are right at the upper rail.
The circuit shown here is capable of monitoring up to The LT6100 is configured for a gain of 50 by grounding
44V applications. Besides the complication of extra parts, both A2 and A4. This is one of the simplest current sens-
the VOS performance of op amps at the supply is gener- ing amplifier circuits where only a sense resistor is re-
ally not factory trimmed, thus less accurate than other quired.
solutions. The finite current gain of the bipolar transistor
is a small source of gain error.

DC-2

Dual LTC6101’s Allow High-Low Current Ranging

CMPZ4697 VLOGIC
(3.3V TO 5V)
10k 7
M1 3
Si4465 +
VIN
4
ILOAD
RSENSE HI –
10m 8 Q1
5 CMPT5551
VOUT
RSENSE LO 40.2k 6
301 100m 301 301 301
4.7k
1.74M
LTC1540
4 3 4 3
2 1 HIGH
2 + – 5 2 + – 5 RANGE
VIN
619k INDICATOR
(ILOAD > 1.2A)

1 1 HIGH CURRENT RANGE OUT
LTC6101 LTC6101
250mV/A

7.5k
VLOGIC

BAT54C
LOW CURRENT RANGE OUT
2.5V/A
R5
7.5k
(VLOGIC +5V) ≤ VIN ≤ 60V
0 ≤ ILOAD ≤ 10A 6101 F03b

Using two current sense amplifiers with two values of rents, less than 1.2 Amps, than with higher currents. A
sense resistors is an easy method of sensing current comparator detects higher current flow, up to 10 Amps,
over a wide range. In this circuit the sensitivity and reso- and switches sensing over to the high current circuitry.
lution of measurement is 10 times greater with low cur-

Two Terminal Current Regulator High Side Power Supply Current Sense

The LT1635 combines an op amp with a 200mV refer-
ence. Scaling this reference voltage to a potential across
resistor R3 forces a controlled amount of current to flow The low offset error of the LTC6800 allows for unusually
from the +terminal to the –terminal. Power is taken from low sense resistance while retaining accuracy.
the loop.

DC-3

0nA to 200nA Current Meter Conventional H-Bridge Current Monitor
100pF BATTERY BUS
+
R1
10M
R4 –
10k
1/2
– 1.5V
LT1495 RS +
INPUT 1/2
CURRENT LT1495 + DIFF
R2 AMP
+ 9k 1.5V IM –
R3
IS = 3µA WHEN IIN = 0
2k
FULL-SCALE NO ON/OFF SWITCH
ADJUST REQUIRED

0µA TO
µA
200µA
1495 TA06
DN374 F03

A floating amplifier circuit converts a full-scale 200nA Many of the newer electric drive functions, such as steer-
flowing in the direction indicated at the inputs to 2V at ing assist, are bidirectional in nature. These functions are
the output of the LT1495. This voltage is converted to a generally driven by H-bridge MOSFET arrays using pulse-
current to drive a 200µA meter movement. By floating width-modulation (PWM) methods to vary the com-
the power to the circuit with batteries, any voltage poten- manded torque. In these systems, there are two main
tial at the inputs are handled. The LT1495 is a micro- purposes for current monitoring. One is to monitor the
power op amp so the quiescent current drain from the current in the load, to track its performance against the
batteries is very low and thus no on/off switch is re- desired command (i.e., closed-loop servo law), and an-
quired. other is for fault detection and protection features.

Over-The-Top Current Sense A common monitoring approach in these systems is to
3V TO 44V
amplify the voltage on a “flying” sense resistor, as
R1 shown. Unfortunately, several potentially hazardous fault
200Ω
3V ground at a motor terminal. Another complication is the
RS
0.2Ω + noise introduced by the PWM activity. While the PWM
LT1637
Q1
2N3904
noise may be filtered for purposes of the servo law, in-
– VOUT formation useful for protection becomes obscured. The
(0V TO 2.7V)
ILOAD
R2 best solution is to simply provide two circuits that indi-
LOAD ILOAD =
VOUT 2k vidually protect each half-bridge and report the bidirec-
(RS)(R2/R1) 1637 TA06
tional load current. In some cases, a smart MOSFET
This circuit is a variation on the “classic” high-side cir- offer the protection features needed. In these situations,
cuit, but takes advantage of Over-the-Top input capability the best solution is the one that derives the load informa-
to separately supply the IC from a low-voltage rail. This tion with the least additional circuitry.
provides a measure of fault protection to downstream
circuitry by virtue of the limited output swing set by the
low-voltage supply. The disadvantage is VOS in the Over-
the-Top mode is generally inferior to other modes, thus

DC-4

Single Supply 2.5V Bidirectional Operation with Fast Current Sense with Alarm
ISENSE
TO RSENSE
CHARGER/
1 8 1µF
FIL– FIL+
–
LT1787
2 VS VS+ 7
2.5V
3 VBIAS 6
DNC
C3
ROUT
4 5 1000pF
VEE
VOUT
–
A1 VOUT A
2.5V +
1M
LT1495 The LT1995 is shown as a simple unity gain difference
5% LT1389-1.25 amplifier. When biased with split supplies the input cur-
1787 F07
The LT1787’s output is buffered by an LT1495 rail-to-rail 100mΩ sense resistor. With 32MHz of bandwidth and
op-amp configured as an I/V converter. This configura- 1000V/usec slew rate the response of this sense ampli-
tion is ideal for monitoring very low voltage supplies. The fier is fast. Adding a simple comparator with a built in
LT1787’s VOUT pin is held equal to the reference voltage reference voltage circuit such as the LT6700-3 can be
appearing at the op amp’s non-inverting input. This al- used to generate an over-current flag. With the 400mV
lows one to monitor supply voltages as low as 2.5V. The reference the flag occurs at 4A.
op-amp’s output may swing from ground to its positive
supply voltage. The low impedance output of the op amp Positive Supply Rail Current Sense
may drive following circuitry more effectively than the VCC
R1
high output impedance of the LT1787. The I/V converter 200Ω

configuration also works well with split supply voltages.
Rs
0.2Ω – –
Battery Current Monitor 1/2 LT1366 Q1 1/2 LT1366
TP0610L
+ +
( )
IL
RSENSE ILOAD R2
CHARGE VO = ILOAD • RS
0.1Ω R1
R2
20k = ILOAD • 20Ω
LOAD
DISCHARGE 5V 12V
1366 TA01
– RA RA
–
A2
1/2 LT1495 RA RA
A1
1/2 LT1495
This is a configuration similar to an LT6100 implemented
+
+ with generic components. A Rail-to-Rail or Over-the-Top
input op amp type is required (for the first section). The
2N3904 2N3904
first section is a variation on the classic high-side where
DISCHARGE
OUT
CHARGE
OUT
VO = IL ()
RB
RA
RSENSE
the P-MOSFET provides an accurate output current into
FOR RA = 1k, RB = 10k
RB RB
VO
R2 (compared to a BJT). The second section is a buffer
= 1V/A
IL 1495 TA05 to allow driving ADC ports, etc., and could be configured
One LT1495 dual op-amp package can be used to estab- with gain if needed. As shown, this circuit can handle up
lish separate charge and discharge current monitoring to 36V operation. Small-signal range is limited by VOL in
outputs. The LT1495 features Over-the-Top operation single-supply operation.

DC-5

LT6100 Load Current Monitor LTC6101 Supply Current
TO LOAD
included as Load in Measurement
RSENSE

+ V+
C1
1 8 0.1µF 5V
V S– V S+ RIN
2 7 RSENSE
VCC A4 4 3
+ C2
3V – +
0.1µF + –
3 6
FIL A2 LOAD 2 5

4 OUT 5
VEE OUTPUT
LT6100
1
6100 F04 LTC6101 VOUT
ROUT
This is the basic LT6100 circuit configuration. The inter-
6101 F06
nal circuitry, including an output buffer, typically operates
from a low voltage supply, such as the 3V shown. The
monitored supply can range anywhere from VCC + 1.4V This is the basic LTC6101 high-side sensing supply-
up to 48V. The A2 and A4 pins can be strapped various monitor configuration, where the supply current drawn
ways to provide a wide range of internally fixed gains. by the IC is included in the readout signal. This configu-
The input leads become very hi-Z when VCC is powered ration is useful when the IC current may not be negligible
down, so as not to drain batteries for example. Access to in terms of overall current draw, such as in low-power
an internal signal node (pin 3) provides an option to in- battery-powered applications. RSENSE should be selected
clude a filtering function with one added capacitor. Small- to limit voltage-drop to <500mV for best linearity. If it is
signal range is limited by VOL in single-supply operation. desirable not to include the IC current in the readout, as
in load monitoring, pin 5 may be connected directly to V+
1A Voltage-Controlled Current Sink instead of the load. Gain accuracy of this circuit is limited
only by the precision of the resistors selected by the user.

V+ Powered Separately from Load Supply

This is a simple controlled current sink, where the op
amp drives the NMOSFET gate to develop a match be-
tween the 1Ω sense resistor drop and the VIN current
command. Since the common-mode voltage seen by the The inputs of the LTC6101 can function from 1.4V above
op amp is near ground potential, a “single-supply” or the device positive supply to 48V DC. In this circuit the
Rail-to-Rail type is required in this application. current flow in the high voltage rail is directly translated
to a 0V to 3V range.

DC-6

Simple High Side Current “Classic” High-Precision Low Side Current Sense
Sense Using the LTC6101 5V

BATTERY BUS
3 5 OUT
+ 3V/AMP
1 LOAD CURRENT
LTC2050HV
RIN 4 IN MEASURED
RSENSE
0.01Ω
100Ω – 2
CIRCUIT, REFERRED
4 3 TO –5V
10Ω 10k
LOAD + –
TO 3mΩ
2 5 MEASURED
CIRCUIT
LOAD CURRENT 0.1µF
– 5V 2050 TA08

1
This configuration is basically a standard non-inverting
VOUT
LT6101
ROUT
4.99V = 10A amplifier. The op amp used must support common-mode
4.99k operation at the lower rail and the use of a Zero-Drift type
VOUT = ILOAD(RSENSE • ROUT/RIN) DN374 F01 (as shown) provides excellent precision. The output of
this circuit is referenced to the lower Kelvin contact,
This is a basic high side current monitor using the
which could be ground in a single-supply application.
LTC6101. The selection of RIN and ROUT establishes the
Small-signal range is limited by VOL for single-supply
desired gain of this circuit, powered directly from the
battery bus. The current output of the LTC6101 allows it designs. Scaling accuracy is set by the quality of the
user-selected resistors.
to be located remotely to ROUT. Thus, the amplifier can
be placed directly at the shunt, while ROUT is placed near
the monitoring electronics without ground drop errors.
This circuit has a fast 1µs response time that makes it
ideal for providing MOSFET load switch protection. The
switch element may be the high side type connected be-
tween the sense resistor and the load, a low side type
between the load and ground or an H-bridge. The circuit
is programmable to produce up to 1mA of full-scale out-
put current into ROUT, yet draws a mere 250µA supply
current when the load is off.

DC-7


Level Shifting
Quite often it is required to sense current flow in a supply V+ Powered Separately from Load Supply
rail that is a much higher voltage potential than the sup-
ply voltage for the system electronics. Current sense cir-
cuits with high voltage capability are useful to translate
information to lower voltage signals for processing.

the Introduction.

Over-The-Top Current Sense
3V TO 44V
R1
200Ω

3V The inputs of the LTC6101 can function from 1.4V above
RS
0.2Ω + the device positive supply to 48V DC. In this circuit the
LT1637
Q1 current flow in the high voltage rail is directly translated
2N3904
– VOUT to a 0V to 3V range.
ILOAD (0V TO 2.7V)
R2
VOUT 2k Voltage Translator
LOAD ILOAD =
(RS)(R2/R1) 1637 TA06

+
RIN
VIN
This circuit is a variation on the “classic” high-side cir- – 4 3
cuit, but takes advantage of Over-the-Top input capability
–
to separately supply the IC from a low-voltage rail. This
+

provides a measure of fault protection to downstream 2 5

circuitry by virtue of the limited output swing set by the +
low-voltage supply. The disadvantage is VOS in the Over- VTRANSLATE –
the-Top mode is generally inferior to other modes, thus 1
less accurate. The finite current gain of the bipolar tran- LTC6101 VOUT
ROUT

This is a convenient usage of the LTC6101 current sense
amplifier as a high voltage level translator. Differential
voltage signals riding on top of a high common mode
voltage (up to 105V with the LTC6101HV) get converted
to a current, through RIN, and then scaled down to a
ground referenced voltage across ROUT.

Level Shifting-1



Level Shifting-2


High Voltage
Monitoring current flow in a high voltage line often re- Measuring bias current into an Avalanche Photo
quires floating the supply of the measuring circuits up Diode (APD) using an instrumentation amplifier.
near the high voltage potentials. Level shifting and isola- 1k
tion components are then often used to develop a lower VIN
1%
BIAS OUTPUT
output voltage indication. 10V TO 33V
35V
TO APD

To see other chapters in this Application Note, return to – CURRENT
the Introduction. 0mA TO 1mA = 0V TO 1V
+ A=1
Over-The-Top Current Sense AN92 F02a

3V TO 44V
1N4684
R1 1k
3.3V
200Ω 1%
VIN BIAS OUTPUT
10V TO 35V TO APD
3V 10M
RS –
0.2Ω + CURRENT
Q1 LT1789 MONITOR OUTPUT
LT1637 0mA TO 1mA = 0V TO 1V
2N3904
+
– VOUT A=1
ILOAD (0V TO 2.7V)
AN92 F02b
R2
VOUT 2k
LOAD ILOAD =
(RS)(R2/R1)
The upper circuit uses an instrumentation amplifier (IA)
1637 TA06
powered by a separate rail (>1V above VIN) to measure
across the 1kΩ current shunt. The lower figure is similar
This circuit is a variation on the “classic” high-side cir- but derives its power supply from the APD bias line. The
cuit, but takes advantage of Over-the-Top input capability limitation of these circuits is the 35V maximum APD
to separately supply the IC from a low-voltage rail. This voltage, whereas some APDs may require 90V or more.
provides a measure of fault protection to downstream In the single-supply configuration shown, there is also a
circuitry by virtue of the limited output swing set by the dynamic range limitation due to VOL to consider. The ad-
low-voltage supply. The disadvantage is VOS in the Over- vantage of this approach is the high accuracy that is
the-Top mode is generally inferior to other modes, thus available in an IA.

High Voltage-1

Simple 500V Current Monitor 48V Supply Current Monitor with
Isolated Output and 105V Survivability

Adding two external Mosfets to hold off the voltage al-
lows the LTC6101 to connect to very high potentials and
monitor the current flow. The output current from the
LTC6101, which is proportional to the sensed input volt-
age, flows through M1 to create a ground referenced
output voltage.

High Voltage-2



High Voltage-3


Low Voltage
To see other chapters in this Application Note, return to 1.25V Electronic Circuit Breaker
the Introduction. SI4864DY
VIN VOUT
1.25V 1.25V
Single Supply 2.5V Bidirectional Operation with 3.5A

External Voltage Reference and I/V Converter SENSEP GATE SENSEN
VBIAS
VCC VBIAS
2.3V TO 6V
ISENSE
TO RSENSE LTC4213
CHARGER/ 10k
LOAD C1 2.5V + VSENSE(MAX) OFF ON ON GND ISEL READY
1 8 1µF
FIL– FIL+
–
LT1787
2 VS VS+ 7 4213 TA01

2.5V
VBIAS 6
3
DNC The LTC4213 provides protection and automatic circuit
ROUT
C3
1000pF
breaker action by sensing Drain-to-Source voltage-drop
4 5
VEE
VOUT
across the NMOSFET. The sense inputs have a Rail-to-
– Rail common mode range, so the circuit breaker can pro-
A1 VOUT A
2.5V + LT1495
tect bus voltages from 0V up to 6V. Logic signals flag a
1M
5% LT1389-1.25 trip condition (with the READY output signal) and reini-
1787 F07 tialize the breaker (using the ON input). The ON input
may also be used as a command in a “smart switch” ap-
plication.
The LT1787’s output is buffered by an LT1495 rail-to-rail
op-amp configured as an I/V converter. This configura-
tion is ideal for monitoring very low voltage supplies. The
LT1787’s VOUT pin is held equal to the reference voltage
appearing at the op amp’s non-inverting input. This al-
lows one to monitor supply voltages as low as 2.5V. The
may drive following circuitry more effectively than the
high output impedance of the LT1787. The I/V converter

Low Voltage-1


High Current (100mA to Amps)
Sensing high currents accurately requires excellent con- Shunt Diode Limits Maximum Input Voltage
trol of the sensing resistance, which is typically a very to Allow Better Low Input Resolution
small value to minimize losses, and the dynamic range of Without Over-Ranging the LTC6101
the measurement circuitry
V+

the Introduction. RSENSE DSENSE

6101 F03a

Kelvin Input Connection Preserves
LOAD
Accuracy Despite Large Load Currents
If low sense currents must be resolved accurately in a
V+
system that has very wide dynamic range, more gain can
be taken in the sense amplifier by using a smaller value
RIN for resistor RIN. This can result in an operating current
RSENSE
4 3
greater than the max current spec allowed unless the
max current is limited in another way, such as with a
+ –
LOAD Schottky diode across RSENSE. This will reduce the high
2 5 current measurement accuracy by limiting the result,
while increasing the low current measurement resolution.
This approach can be helpful in cases where an occa-
1
sional large burst of current may be ignored.
LTC6101 VOUT
ROUT
Kelvin Sensing
6101 F02
DIRECTION OF CHARGING CURRENT

Kelvin connection of the IN– and IN+ inputs to the sense
resistor should be used in all but the lowest power appli- RSENSE
cations. Solder connections and PC board interconnec-
tions that carry high current can cause significant error in 4008 F12

measurement due to their relatively large resistances. By
isolating the sense traces from the high current paths, CSP BAT
this error can be reduced by orders of magnitude. A
In any high current, >1Amp, application, Kelvin contacts
sense resistor with integrated Kelvin sense terminals will
to the sense resistor are important to maintain accuracy.
give the best results.
This simple illustration from a battery charger application
shows two voltage-sensing traces added to the pads of
the current sense resistor. If the voltage is sensed with
high impedance amplifier inputs, no IxR voltage drop
errors are developed.

High Current (100mA to Amps)-1

0A to 33A High Side Current Monitor with Filtering Single Supply RMS Current Measurement
4.4V TO 48V 3V
SUPPLY
2 7 6
LT6100 VCC A4 A2

+
8 VS

RSENSE VOUT 5 VOUT = 2.5V
3mΩ ISENSE = 33A
–
1 VS

VEE FIL
LOAD 6100 TA01a

4 3
CONFIGURED FOR GAIN = 25V/V 220pF

The LT1966 is a true RMS-to-DC converter that takes a
High current sensing on a high voltage supply rail is eas-
single-ended or differential input signal with rail-to-rail
ily accomplished with the LT6100. The sense amplifier is
range. The output of a pcb mounted current sense trans-
biased from a low 3V supply and pin-strapped to a gain
former can be connected directly to the converter. Up to
of 25V/V to output a 2.5V full scale reading of the current
75A of AC current is measurable without breaking the
flow. A capacitor at the FIL pin to ground will filter out
signal path from a power source to a load. The accurate
noise of the system (220pF produces a 12KHz low pass
operating range of the circuit is determined by the selec-
corner frequency).
tion of the transformer termination resistor. All of the
math is built in to the LTC1966 to provide a dc output
voltage that is proportional to the true rms value of the
current. This is valuable in determining the power/energy
consumption of ac powered appliances.


CMPZ4697 VLOGIC
(3.3V TO 5V)
10k 7
M1 3
Si4465 +
VIN
4
ILOAD
RSENSE HI –
10m 8 Q1
5 CMPT5551
VOUT
RSENSE LO 40.2k 6
301 100m 301 301 301
4.7k
1.74M
LTC1540
4 3 4 3
2 1 HIGH
2 + – 5 2 + – 5 RANGE
VIN
619k INDICATOR
(ILOAD > 1.2A)

LTC6101 LTC6101
250mV/A

7.5k
VLOGIC

BAT54C
2.5V/A
R5
7.5k
0 ≤ ILOAD ≤ 10A 6101 F03b



LDO Load Balancing
VIN BALLAST RESISTANCE:
IN OUT
1.8V TO 20V + IDENTICAL LENGTH
10µF LT1763 0.01µF 10µF THERMALLY MATED
SHDN BYP WIRE OR PCB TRACE
FB
R2 R1
2k 2k

⎛ R1⎞
IN OUT VOUT = 1.22V ⎜1 + ⎟
⎝ R2⎠
LT1763 0.01µF 10µF
SHDN BYP 100Ω
FB ILOAD LOAD
2k
2k
1k
0.1µF
–
A
10k
+

IN OUT
LT1763 0.01µF 10µF
SHDN BYP 100Ω
FB
2k
2k
0 ≤ ILOAD ≤ 1.5A 1k
1.22V ≤ VOUT ≤ VDD 0.1µF
LDO LOADS MATCH TO WITHIN VDD
1mA WITH 10mΩ OF BALLAST –
RESISTANCE (2 INCHES OF AWG
28 GAUGE STRANDED WIRE) B
A, B: LTC6078 10k
+ 60789 TA09

As system design enhancements are made there is often and servo’ed to match the master regulator output volt-
the need to supply more current to a load than originally age. The precise low offset voltage of the LTC6078 dual
expected. A simple way to modify power amplifiers or op amp (10uV) balances the load current provided by
voltage regulators, as shown here, is to parallel devices. each regulator to within 1mA. This is achieved using a
When paralleling devices it is desired that each device very small 10mΩ current sense resistor in series with
shares the total load current equally. In this circuit two each output. This sense resistor can be implemented
adjustable “slave” regulator output voltages are sensed with pcb copper traces or thin gauge wire.


Sensing Output Current
VCC
0V TO 1V
12V
VCSRC
VCSNK
EN
+IN VCC
V+
ISRC
ISNK RS
TSD 0.2Ω
LT1970 OUT
SENSE+
SENSE–
RLOAD
FILTER
–
–IN V
VEE
COMMON

LT1787 R4
255k
–12V VS– VS+
RG RF BIAS
–12V 12V
R1
20k
60.4k –
VEE VOUT
R2 LT1880 2.5V
10k + ±5mV/mA

R3 1kHz FULL CURRENT
–12V 20k –12V BANDWIDTH

0V TO 5V A/D

1970 F10

OPTIONAL DIGITAL FEEDBACK

The LT1970 is a 500mA power amplifier with voltage in a microprocessor controlled system. For closed loop
programmable output current limit. Separate DC voltage control of the current to a load an LT1787 can monitor
inputs and an output current sensing resistor control the the output current. The LT1880 op amp provides scaling
maximum sourcing and sinking current values. These and level shifting of the voltage applied to an A-to-D
control voltages could be provided by a D-to-A Converter Converter for a 5mV/mA feedback signal.



Low Current (Picoamps to Milliamps)
For low current applications the easiest way to sense cur- Gain of 50 Current Sense
rent is to use a large sense resistor. This however causes ISENSE RSENSE
larger voltage drops in the line being sensed which may VSUPPLY
6.4V TO 48V
not be acceptable. Using a smaller sense resistor and +
LT6100 VS VS– LOAD
taking gain in the sense amplifier stage is often a better
approach. Low current implies high source impedance –
+
measurements which are subject approach. Low current
implies high source impedance measurements which are 5V VCC
subject to noise pickup and often require filtering of
FIL
some sort. VOUT
50 • RSENSE • ISENSE
VEE A2 A4
To see other chapters in this Application Note, return to 6100 TA04

the Introduction.

Filtered Gain of 20 Current Sense The LT6100 is configured for a gain of 50 by grounding
both A2 and A4. This is one of the simplest current sens-
ISENSE
VSUPPLY
RSENSE ing amplifier circuits where only a sense resistor is re-
4.4V TO 48V quired.
+
LT6100 VS V S– LOAD

0nA to 200nA Current Meter
+ –
100pF

3V VCC

R1
FIL 10M
1000pF VOUT
20 • RSENSE • ISENSE R4 –
VEE A2 A4 10k
1/2
6100 TA03
– 1.5V
LT1495
–3dB AT 2.6kHz INPUT 1/2
CURRENT LT1495 +
R2
The LT6100 has pin strap connections to establish a vari- + 9k 1.5V

ety of accurate gain settings without using external com- R3
IS = 3µA WHEN IIN = 0
2k
ponents. For this circuit grounding A2 and leaving A4 FULL-SCALE NO ON/OFF SWITCH
ADJUST REQUIRED
open set a gain of 20. Adding one external capacitor to
0µA TO
the FIL pin creates a low-pass filter in the signal path. A µA
200µA
capacitor of 1000pF as shown sets a filter corner fre- 1495 TA06

quency of 2.6KHz. A floating amplifier circuit converts a full-scale 200nA
flowing in the direction indicated at the inputs to 2V at
the output of the LT1495. This voltage is converted to a
current to drive a 200µA meter movement. By floating
the power to the circuit with batteries, any voltage poten-
tial at the inputs are handled. The LT1495 is a micro-
power op amp so the quiescent current drain from the
batteries is very low and thus no on/off switch is re-
quired.

Low Current (Picoamps to Milliamps)-1

Lock-In Amplifier Technique Permits 1% Accurate APD Current Measurement Over 100nA to 1mA Range.
FOR OPTIONAL “ZERO CURRENT” FEEDBACK TO
1k* APD BIAS REGULATOR, SEE APPENDIX A
APD 1%
VOUT = 20V TO 90V
HIGH VOLTAGE TO APD
BIAS INPUT 1µF 1µF
100V 100k* 100k* 100V
Q1

1N4690 1M*
5.6V 5V
0.2µF 5V

– 1µF 6
A1
20k 2 +
S2 OUTPUT
10k LT1789 A2
0V TO 1V =
30k 1µF LT1006
+ 5 0mA TO 1mA
–
Q2 0.2µF 20k*
MPSA42 1M* –3.5V –3.5V
20k

12 200k*
13 14
S1 –3.5V TO
5V 18 AMPLIFIERS
22µF
5V 3
S3

+
* = 0.1% METAL FILM RESISTOR
15 22µF
1µF 100V = TECATE CMC100105MX1825 +
# CIRCLED NUMBERS = LTC1043 PIN NUMBER
= 1N4148
= TP0610L 16 17 4

0.056µF

5V
AN92 F04

Avalanche Photodiodes, APDs, require a small amount of which feeds A1 through 0.2µF AC coupling capacitors.
current from a high voltage supply. The current into the A1’s single-ended output biases demodulator S2, which
diode is an indication of optical signal strength and must presents a DC output to buffer amplifier A2. A2’s output
be monitored very accurately. It is desirable to power all is the circuit output.
of the support circuitry from a single 5V supply.
Switch S3 clocks a negative output charge pump which
This circuit utilizes AC carrier modulation techniques to supplies the amplifier’s V– pins, permitting output swing
meet APD current monitor requirements. It features to (and below) zero volts. The 100k resistors at Q1
0.4% accuracy over the sensed current range, runs from minimize its on-resistance error contribution and prevent
a 5V supply and has the high noise rejection character destructive potentials from reaching A1 (and the 5V rail)
stics of carrier based “lock in” measurements. if either 0.2µF capacitor fails. A2’s gain of 1.1 corrects for
the slight attenuation introduced by A1’s input resistors.
The LTC1043 switch array is clocked by its internal oscil- In practice, it may be desirable to derive the APD bias
lator. Oscillator frequency, set by the capacitor at Pin 16, voltage regulator’s feedback signal from the indicated
is about 150Hz. S1 clocking biases Q1 via level shifter point, eliminating the 1kΩshunt resistor’s voltage drop.
Q2. Q1 chops the DC voltage across the 1k current Verifying accuracy involves loading the APD bias line
shunt, modulating it into a differential square wave signal with 100nA to 1mA and noting output agreement.


DC Coupled APD Current Monitor
FOR OPTIONAL “ZERO CURRENT” FEEDBACK TO
1N4690 1k* APD BIAS REGULATOR, SEE APPENDIX A
APD 5.6V CURRENT SHUNT
VOUT = 20V TO 90V
HIGH VOLTAGE TO APD
BIAS INPUT
10M 1k* 51K

+ + 1N4702
A1 1µF 15V
51k LT1077
–
Q1 100k
ZVP0545A Q2
5V
MPSA42
10k LT1460
1k* 5V
Hi-Z OUTPUT 2.5V
0V TO 1V = 0mA TO 1mA
1k*
VIN VREF FO

BUFFERED OUTPUT LTC2400 SCK
5V 0mA TO 1mA = 0V TO 1V 5V A-TO-D DIGITAL
SDO
* = 0.1% METAL FILM RESISTOR + INTERFACE
1k 10µF CS
= BAT85 A2
LTC1150 OPTIONAL

+
– CLK OUT 39k
Q2 10µF
DIGITAL OUTPUT

V– 2N3904 +
100k
≈ –3.5V HERE
OPTIONAL BUFFERED OUTPUT
AN92 F05

Avalanche Photodiodes, APDs, require a small amount of across the 20V to 90V APD bias voltage range. The 5.6V
current from a high voltage supply. The current into the zener assures A1’s inputs are always within their com-
diode is an indication of optical signal strength and must mon mode operating range and the 10MΩ resistor main-
be monitored very accurately. It is desirable to power all tains adequate zener current when APD current is at very
of the support circuitry from a single 5V supply. low levels.

This circuit’s DC coupled current monitor eliminates the Two output options are shown. A2, a chopper stabilized
previous circuit’s trim but pulls more current from the amplifier, provides an analog output. Its output is able to
APD bias supply. A1 floats, powered by the APD bias rail. swing to (and below) zero because its V– pin is supplied
The 15V zener diode and current source Q2 ensure A1 with a negative voltage. This potential is generated by
never is exposed to destructive voltages. The 1kΩ cur- using A2’s internal clock to activate a charge pump
rent shunt’s voltage drop sets A1’s positive input poten- which, in turn, biases A2’s V– pin.3 A second output op-
tial. A1 balances its inputs by feedback controlling its tion substitutes an A-to-D converter, providing a serial
negative input via Q1. As such, Q1’s source voltage format digital output. No V– supply is required, as the
equals A1’s positive input voltage and its drain current LTC2400 A-to-D will convert inputs to (and slightly be-
sets the voltage across its source resistor. Q1’s drain cur- low) zero volts.
rent produces a voltage drop across the ground referred
1kΩ resistor identical to the drop across the 1kΩ current
shunt and, hence, APD current. This relationship holds


Six Decade (10nA to 10mA) Current Log Amplifier

–
C

100Ω
+
–
B
100Ω
+
33µF

Q1 Q2
100k
133k
VDD

– 1000pF
A –
1.58k
+ D
PRECISION
IIN + VOUT RESISTOR PT146
1k
VCC LT6650
+3500ppm/°C

IN OUT 60789 TA07
GND 10nA ≤ IIN ≤ 10mA
1µF 1µF
Q1, Q2: DIODES INC. DMMT3906W
A TO D: LTC6079
VOUT ≈ 150mV • log (IIN) + 1.23V, IIN IN AMPS

Using precision quad amplifiers like the LTC6079, (10µV
offset and <1pA bias current) allow for very wide range
current sensing. In this circuit a six decade range of cur-
rent pulled from the circuit input terminal is converted to
an output voltage in logarithmic fashion increasing
150mV for every decade of current change.



Motors and Inductive Loads
The largest challenge in measuring current through in- Conventional H-Bridge Current Monitor
ductive circuits is the transients of voltage that often oc- BATTERY BUS
cur. Current flow can remain continuous in one direction +
while the voltage across the sense terminals reverses in
polarity.

the Introduction. RS +
DIFF
AMP
Electronic Circuit Breaker IM –

ON/OFF IN VS
CT CD RD *RSEN
0.22µF 0.01µF 100k 0.1Ω
CT DS
Z5U
LTC1153 DN374 F03

TO µP STATUS G IRLR024
51k
GND SHUTDOWN
51k ing assist, are bidirectional in nature. These functions are
5V
SENSITIVE
generally driven by H-bridge MOSFET arrays using pulse-
**70°C
PTC 5V LOAD width-modulation (PWM) methods to vary the com-
manded torque. In these systems, there are two main
ALL COMPONENTS SHOWN ARE SURFACE MOUNT.
purposes for current monitoring. One is to monitor the
* IMS026 INTERNATIONAL MANUFACTURING SERVICE, INC. (401) 683-9700 current in the load, to track its performance against the
** RL2006-100-70-30-PT1 KEYSTONE CARBON COMPANY (814) 781-1591
LTC1153 • TA01
desired command (i.e., closed-loop servo law), and an-
other is for fault detection and protection features.
The LTC1153 is an Electronic Circuit Breaker. Sensed cur-
rent to a load opens the breaker when 100mV is devel- A common monitoring approach in these systems is to
oped between the supply input, Vs, and the Drain Sense amplify the voltage on a “flying” sense resistor, as
pin, DS. To avoid transient, or nuisance trips of the break shown. Unfortunately, several potentially hazardous fault
components RD and CD delay the action for 1msec. A scenarios go undetected, such as a simple short to
thermistor can also be used to bias the Shutdown input ground at a motor terminal. Another complication is the
to monitor heat generated in the load and remove power noise introduced by the PWM activity. While the PWM
should the temperature exceed 70°C in this example. A noise may be filtered for purposes of the servo law, in-
feature of the LTC1153 is timed Automatic Reset which formation useful for protection becomes obscured. The
will try to re-connect the load after 200msec using the best solution is to simply provide two circuits that indi-
0.22µF timer capacitor shown. vidually protect each half-bridge and report the bidirec-
offer the protection features needed. In these situations,

Motors and Inductive Loads-1

Motor Speed Control Practical H-Bridge Current Monitor Offers Fault
OV TO 5V
Detection and Bidirectional Load Information
TORQUE/STALL
CURRENT CONTROL
15V –
VCSRC
BATTERY BUS DIFF
VCSNK
OUTPUT
EN TO ADC
+IN VCC
V+ +
ISRC LTC6101 RIN RIN LTC6101
ISNK RS
1Ω ROUT ROUT
TSD RS RS
LT1970 OUT
SENSE+
SENSE–
12V DC
+
FILTER MOTOR
–IN V– FOR IM RANGE = ±100A,
VEE
DIFF OUT = ±2.5V
COMMON GND
15V
RS = 1mΩ
C1
RIN = 200Ω
R1 –15V 1µF TACH ROUT = 4.99k
1.2k
FEEDBACK
REVERSE
3V/1000rpm IM
R4 R5
49.9k 49.9k
R2 1970 F13

10k
FORWARD

R3
DN374 F04
1.2k

–15V This circuit implements a differential load measurement
This uses an LT1970 power amplifier as a linear driver of for an ADC using twin unidirectional sense measure-
a DC motor with speed control. The ability to source and ments. Each LTC6101 performs high side sensing that
sink the same amount of output current provides for bi- rapidly responds to fault conditions, including load
directional rotation of the motor. Speed control is man- shorts and MOSFET failures. Hardware local to the switch
aged by sensing the output of a tachometer built on to module (not shown in the diagram) can provide the pro-
the motor. A typical feedback signal of 3V/1000rpm is tection logic and furnish a status flag to the control sys-
compared with the desired speed-set input voltage. Be- tem. The two LTC6101 outputs taken differentially pro-
cause the LT1970 is unity-gain stable, it can be config- duce a bidirectional load measurement for the control
ured as an integrator to force whatever voltage across servo. The ground-referenced signals are compatible
the motor as necessary to match the feedback speed with most ∆ΣADCs. The ∆ΣADC circuit also provides a
signal with the set input signal. Additionally, the current “free” integration function that removes PWM content
limit of the amplifier can be adjusted to control the torque from the measurement. This scheme also eliminates the
and stall current of the motor. need for analog-to-digital conversions at the rate needed
to support switch protection, thus reducing cost and
complexity.


Lamp Driver Relay Driver
12V 12V +
+ 100µF
470µF
10k 0.02Ω 2Ω 0.02Ω
IN VS
100k 10k
IN VS
5V
CT DS 0.01µF
5V 1N4148
0.33µF LTC1153 VN2222LL CT DS
STATUS G 1µF LTC1153 MTD3055E
0.1µF
1M STATUS G TO 12V
LOAD
GND SD IRFZ34 15V
12V GND SD

12V/2A 1N4001
BULB COIL CURRENT LIMITED TO 350mA
CONTACT CURRENT LIMITED TO 5A
LTC1153 • TA07 LTC1153 • TA08

The inrush current created by a lamp during turn-on can This circuit provides reliable control of a relay by using
be 10 to 20 times greater than the rated operating cur- an Electronic Circuit Breaker circuit with two-level over-
rent. This circuit shifts the trip threshold of an LTC1153 current protection. Current flow is sensed through two
Electronic Circuit Breaker up by a factor of 11:1 (to 30A) separate resistors, one for the current into the relay coil
for 100ms while the bulb is turned on. The trip threshold and the other for the current through the relay contacts.
then drops down to 2.7A after the inrush current has When 100mV is developed between the Vs supply pin
subsided. and the Drain Sense pin, DS, the N-channel MOSFET is
turned off opening the contacts. As shown, the relay coil
Intelligent High Side Switch current is limited to 350mA and the contact current to 5
Amps.

The LT1910 is a dedicated high side MOSFET driver with
built in protection features. It provides the gate drive for
a power switch from standard logic voltage levels. It pro-
vides shorted load protection by monitoring the current
flow to through the switch. Adding an LTC6101 to the
same circuit, sharing the same current sense resistor,


Full-Bridge Load Current Monitor
+VSOURCE 5V
LT1990
900k 10k 8
7

1M 100k
2
–
RS 6
VOUT
– + 3 1M
+
IL VREF = 1.5V
4
10k 5
IN OUT 54.9k 1nF
LT6650 40k 900k
GND FB 40k 100k

20k
–12V ≤ VCM ≤ 73V
VOUT = VREF ± (10 • IL • RS) 1 1990 TA01

1µF

The LT1990 is a difference amplifier that features a very the output away from ground. The output will move
wide common mode input voltage range that can far ex- above or below 1.5V as a function of which direction the
ceed its own supply voltage. This is an advantage to re- current in the load is flowing. As shown, the amplifier
ject transient voltages when used to monitor the current provides a gain of 10 to the voltage developed across
in a full bridge driven inductive load such as a motor. The resistor RS.
LT6650 provides a voltage reference of 1.5V to bias up



Batteries
The science of battery chemistries and the charging and Charge/Discharge Current Monitor
discharging characteristics is a book of its own. This on Single Supply with Shifted VBIAS
chapter is intended to provide a few examples of TO RSENSE
monitoring current flow into and out of batteries of any CHARGER/
3.3V
LOAD C1
chemistry. 1 8 1µF
TO
60V
FIL– FIL+ 3.3V
LT1787HV
VS– VS+
To see other chapters in this Application Note, return to 2 7 20k
5%
VBIAS 6
the Introduction. 3
DNC
ROUT C2
4 5 1µF LT1634-1.25
Input Remains Hi-Z when LT6100 is Powered Down VEE
VOUT
C3*
ISENSE 1000pF
RSENSE
TO LOAD *OPTIONAL OUTPUT 1787 F04

+
BATTERY
LT6100 VS
–
V S+
4.1V TO 48V Here the LT1787 is used in a single supply mode with the
POWER
VBIAS pin shifted positive using an external LT1634 volt-
DOWN OK – + age reference. The VOUT output signal can swing above
VCC
3V and below VBIAS to allow monitoring of positive or nega-
VCC
0V
INPUTS
tive currents through the sense resistor. The choice of
REMAIN FIL reference voltage is not critical except for the precaution
Hi-Z
VOUT that adequate headroom must be provided for VOUT to
VEE A2 A4
6100 F08
swing without saturating the internal circuitry. The com-
ponent values shown allow operation with VS supplies as
low as 3.1V.
This is the typical configuration for an LT6100, monitor-
ing the load current of a battery. The circuit is powered Battery Current Monitor
from a low-voltage supply rail rather than the battery be-
IL
ing monitored. A unique benefit of this configuration is CHARGE
RSENSE
0.1Ω
that when the LT6100 is powered down, its battery sense
inputs remain high impedance, drawing less than 1uA of DISCHARGE 5V 12V

current. This is due to an implementation of Linear Tech- – RA RA
–
nology’s Over-The-Top® input technique at its front end. A2 A1
1/2 LT1495 RA RA 1/2 LT1495
+
+

2N3904 2N3904
DISCHARGE
OUT
CHARGE
OUT
VO = IL ()
RB
RA
RSENSE

VO
= 1V/A
IL 1495 TA05


Batteries-1

Input Current Sensing Application Coulomb Counter
5V CHARGER
+ C1 RSENSE
22µF 1µF + LOAD
RP1
3k C2
1 8 1µF
SENSE AVG 1% 4.7µF
2 7
IOUT PROG RL RL
RP2
3 LT1620MS8 6 12k SENSE – SENSE + VDD
GND VCC
1% CF+ INT
4.7µF
4 5 LTC4150 CLR CHG µP
IN – IN+ CF– DISCHG
R1 POL
0.033Ω SHDN
TO GND
+ SYSTEM LOAD
4150 TA01a
22µF
L1B
10µH
MBRS340
The LTC4150 is a micropower high-side sense circuit that
VBATT = 12.3V
7
VIN VSW
5
includes a V/F function. Voltage across the sense resistor
4.7µF L1A 57k is cyclically integrated and reset to provide digital transi-
LT1513 10µH +
6 2
22µF
Li-ION
tions that represent charge flow to or from the battery. A
RUN S/S VFB ×2
4
GND IFB
3 24Ω
6.4k
polarity bit indicates the direction of the current. Supply
GND potential for the LTC4150 is 2.7V to 8.5V. In the free-
TAB VC
8 1 0.22µF RSENSE running mode (as shown, with CLR & INT connected
0.1Ω
0.1µF together) the pulses are approximately 1µs wide and
X7R
around 1Hz full-scale.
1620/21 • F04

Li-Ion Gas Gauge
The LT1620 is coupled with an LT1513 SEPIC battery
charger IC to create an input over current protected POWER-DOWN
SWITCH

charger circuit. The programming voltage (VCC – VPROG) 2.5V CL
LOAD

47µF
is set to 1.0V through a resistor divider (RP1 and RP2)
RL RL
from the 5V input supply to ground. In this configuration, 1 10
3k 3k
SENSE +
if the input current drawn by the battery charger com- RSENSE
INT
LTC4150 CLR
9
0.1Ω 8
bined with the system load requirements exceeds a cur- 2
3
SENSE – VDD
C2
2-CELL + 4.7µF
C F+ 7
rent limit threshold of 3A, the battery charger current will Li-Ion
6V ~ 8.4V CF
GND
µP

be reduced by the LT1620 such that the total input supply 4.7µF
4
C F–
5 6
current is limited to 3A. SHDN POL
SHUTDOWN

This is the same as the Coulomb Counter circuit, except
that the microprocessor clears the integration cycle
complete condition with software, so that a relatively
slow polling routine may be used.

Batteries-2

NiMH Charger
Q3
INPUT SWITCH
DCIN
0V TO 20V
R8 C1
147k 0.1µF
0.25%
VLOGIC BATMON DCIN
RCL
R11 R12 C4
VFB INFET 0.02Ω
100k 100k 0.1µF
1% SYSTEM
ICL ICL LTC4008 CLP R1 5.1k 1%
LOAD
C2
ACP ACP/SHDN CLN RSENSE
20µF
L1 0.025Ω
FAULT FAULT TGATE Q1 10µH 1%
FLAG FLAG BGATE NiMH
R10 32.4k 1% BATTERY
NTC PGND Q2 D1 C3 PACK
20µF
RT CSP
ITH BAT R4 3.01k 1%
R9 R7 GND PROG R5 3.01k 1%
C7 13.3k 6.04k CHARGING
0.47µF 0.25% 1% CURRENT
C5 MONITOR
THERMISTOR RT 0.0047µF D1: MBRS130T3
C6
10k 150k R6 Q1: Si4431ADY
0.12µF
NTC 26.7k Q2: FDC645N
1%

4008 TA02

The LTC4008 is a complete NiMH battery pack controller. connected the battery pack is always kept charged and
It provides automatic switchover to battery power when ready for duty.
the external DC power source is removed. When power is

Single Cell Li-Ion Charger Li-Ion Charger
VIN 800mA (WALL)
5V TO 22V WALL LTC4076 500mA (USB)
ADAPTER DCIN BAT
USB USBIN HPWR + 4.2V
PORT 1µF SINGLE CELL
IUSB
0.1µF 10µF Li-Ion BATTERY
VCC 2k
BAT 1µF IDC ITERM
GATE 1% 1.24k GND 1k
2k 1% 1%
LTC4002ES8-4.2
CHARGE 6.8µH 4076 TA01
STATUS
CHRG SENSE
Just a few external components are required for this sin-
68mΩ
gle Li-Ion cell charger. Power for the charger can come
COMP BAT
NTC GND
from a wall adapter or a computer’s USB port.
0.47µF + Li-Ion
22µF
BATTERY
2.2k 10k
T 4002 TA01
NTC
NTC: DALE NTHS-1206N02

Controlling the current flow in Lithium-Ion battery charg-
ers is essential for safety and extending useful battery
life. Intelligent battery charger ICs can be used in fairly
simple circuits to monitor and control current, voltage
and even battery pack temperature for fast and safe
charging.

Batteries-3

Battery Monitor
RS RA
0.2Ω 2k Q1
CHARGER
VOLTAGE
+ 2N3904
A
IBATT RA' 1/4 LT1491 –
2k C
– 1/4 LT1491 LOGIC
+
RB
2k Q2
+ 2N3904 LOGIC HIGH (5V) = CHARGING
B LOGIC LOW (0V) = DISCHARGING
RB' 1/4 LT1491
2k
LOAD – +
+ D
RG VOUT
1/4 LT1491
10k
VBATT = 12V
–
S1 10k 90.9k

VOUT V S1 = OPEN, GAIN = 1 RA = R B
IBATT = = OUT AMPS
(RS)(RG /RA)(GAIN) GAIN S1 = CLOSED, GAIN = 10 VS = 5V, 0V 1490/91 TA01

Op-amp sections A & B form classical high-side sense cating whether the current is a charge or discharge flow.
circuits in conjunction with Q1 & Q2 respectively. Each S1 sets the section D buffer op-amp gain to +1 or +10.
section handles a different polarity of battery current flow Rail-to-Rail op-amps are required in this circuit, such as
and delivers metered current to load resistor RG. Section the LT1491 quad in the example.
C operates as a comparator to provide a logic signal indi-

Batteries-4


High Speed
Current monitoring is not normally a particularly high To see other chapters in this Application Note, return to
speed requirement unless excessive current flow is the Introduction.
caused by a fault of some sort. The use of fast amplifiers
in conventional current sense circuits is usually sufficient
to obtain the response time desired.

ISENSE = 0A TO 30A
ACCURACY ≈ 3%
VOUT
Q1 R1 1k
FMMT493 4.7k 1%
VS = 3V
30.1Ω
1% –
0805
LT1797
×3
+
0.1µF
VZ = 6.8V 0.003Ω
1% 3W
(–42V TO –56V) – + 1797 TA01

ISENSE


High Speed-1

Conventional H-Bridge Current Monitor Single Supply 2.5V Bidirectional Operation with
BATTERY BUS
+ ISENSE
TO RSENSE
CHARGER/
1 8 1µF
FIL– FIL+
–
LT1787
RS + 2 VS VS+ 7
DIFF 2.5V
3 VBIAS 6
AMP DNC
IM – ROUT
C3
4 5 1000pF
VEE
VOUT
–
A1 VOUT A
2.5V + LT1495
1M
DN374 F03
5% LT1389-1.25
1787 F07

generally driven by H-bridge MOSFET arrays using pulse- The LT1787’s output is buffered by an LT1495 rail-to-rail
width-modulation (PWM) methods to vary the com- op-amp configured as an I/V converter. This configura-
manded torque. In these systems, there are two main tion is ideal for monitoring very low voltage supplies. The
purposes for current monitoring. One is to monitor the LT1787’s VOUT pin is held equal to the reference voltage
current in the load, to track its performance against the appearing at the op amp’s non-inverting input. This al-
desired command (i.e., closed-loop servo law), and an- lows one to monitor supply voltages as low as 2.5V. The
other is for fault detection and protection features. op-amp’s output may swing from ground to its positive
A common monitoring approach in these systems is to may drive following circuitry more effectively than the
amplify the voltage on a “flying” sense resistor, as high output impedance of the LT1787. The I/V converter
shown. Unfortunately, several potentially hazardous fault configuration also works well with split supply voltages.
ground at a motor terminal. Another complication is the Battery Current Monitor
noise introduced by the PWM activity. While the PWM IL
RSENSE
CHARGE
noise may be filtered for purposes of the servo law, in- 0.1Ω

formation useful for protection becomes obscured. The
DISCHARGE 5V 12V
best solution is to simply provide two circuits that indi- RA RA
–
–
A2 A1
tional load current. In some cases, a smart MOSFET 1/2 LT1495 RA RA 1/2 LT1495
+
+
offer the protection features needed. In these situations, 2N3904 2N3904
DISCHARGE
OUT
CHARGE
OUT
VO = IL ()
RB
RA
RSENSE

VO
= 1V/A
IL 1495 TA05


High Speed-2

Fast Current Sense with Alarm Fast Differential Current Source

15V
R* 2 10pF R* VIN2 – VIN1
7
VIN1 – IOUT =
R
6
LT1022
R* 3
VIN2 + 4
R*
–15V IOUT RL

*MATCH TO 0.01%
FULL-SCALE POWER BANDWIDTH
= 1MHz FOR IOUTR = 8VP-P
= 400kHz FOR IOUTR = 20VP-P
MAXIMUM IOUT = 10mAP-P IOUTP-P • RL
COMMON-MODE VOLTAGE AT LT1022 INPUT =
The LT1995 is shown as a simple unity gain difference 2

amplifier. When biased with split supplies the input cur- LT1022 • TA07

rent can flow in either direction providing an output volt- This is a variation on the Howland configuration, where
age of 100mV per Amp from the voltage across the load current actually passes through a feedback resistor
100mΩ sense resistor. With 32MHz of bandwidth and as an implicit sense resistance. Since the effective sense
1000V/usec slew rate the response of this sense ampli- resistance is relatively large, this topology is appropriate
fier is fast. Adding a simple comparator with a built in for producing small controlled currents.
reference voltage circuit such as the LT6700-3 can be
used to generate an over-current flag. With the 400mV
reference the flag occurs at 4A.

High Speed-3


Fault Sensing
The lack of current flow or the dramatic increase of cur- Schottky Prevents Damage During Supply Reversal
rent flow very often indicates a system fault. In these cir- RSENSE
cuits it is important to not only detect the condition, but
R1
also ensure the safe operation of the detection circuitry 100
itself. System faults can be destructive in many unpre- 4 3
dictable ways. L 2 + – 5
O
A
D VBATT
the Introduction. LTC6101
1

High Side Current Sense and Fuse Monitor D1 R2
4.99k
RSENSE 6101 F07
TO LOAD 2mΩ FUSE
BATTERY
BUS
1 8 +
The LTC6101 is not protected internally from external
VS– VS+
ADC 2 7
reversal of supply polarity. To prevent damage that may
POWER VCC A4
≥2.7V C2
occur during this condition, a Schottky diode should be
–
added in series with V–. This will limit the reverse current
+
0.1µF
3
FIL A2
6 through the LTC6101. Note that this diode will limit the
low voltage performance of the LTC6101 by effectively
4 OUT 5 OUTPUT
reducing the supply voltage to the part by VD.
VEE 2.5V = 25A
LT6100
DN374 F02
Additional Resistor R3 Protects
Output During Supply Reversal
The LT6100 can be used as a combination current sensor
RSENSE
and fuse monitor. This part includes on-chip output buff-
ering and was designed to operate with the low supply R1
VBATT
100
voltage (≥2.7V), typical of vehicle data acquisition sys-
4 3
tems, while the sense inputs monitor signals at the L 2 + – 5
higher battery bus potential. The LT6100 inputs are toler- O
A
ant of large input differentials, thus allowing the blown- D
R3
fuse operating condition (this would be detected by an LTC6101
1 1k
ADC
output full-scale indication). The LT6100 can also be
D1 R2
powered down while maintaining high impedance sense 4.99k
6101 F08
inputs, drawing less than 1µA max from the battery bus.

If the output of the LTC6101 is wired to an independently
powered device that will effectively short the output to
another rail or ground (such as through an ESD protec-
tion clamp) during a reverse supply condition, the
LTC6101’s output should be connected through a resistor
or Schottky diode to prevent excessive fault current.

Fault Sensing-1

Electronic Circuit Breaker 1.25V Electronic Circuit Breaker
SI4864DY
VIN VOUT
0.033Ω Si9434DY 5V AT 1A
1.25V 1.25V
5V PROTECTED 3.5A
0.1µF 1k

VBIAS SENSEP GATE SENSEN
FAULT VCC VBIAS
2.3V TO 6V

CDELAY
LTC4213
10k
100Ω
1 8 1N4148 OFF ON ON GND ISEL READY
33k SENSE AVG
2 7
2N3904 IOUT PROG 100k
3 LT1620MS8 6 4.7k 33k 4213 TA01
GND VCC

4
–IN +IN
5 The LTC4213 provides protection and automatic circuit
breaker action by sensing Drain-to-Source voltage-drop
TYPICAL DC TRIP AT 1.6A 2N3904
3A FAULT TRIPS
LT1620/21 • TA03
across the NMOSFET. The sense inputs have a Rail-to-
IN 2ms WITH CDELAY = 1.0µF
Rail common mode range, so the circuit breaker can pro-
The LT1620l current sense amplifier is used to detect an tect bus voltages from 0V up to 6V. Logic signals flag a
over-current condition and shut off a P-MOSFET load trip condition (with the READY output signal) and reini-
switch. A fault flag is produced in the over-current condi- tialize the breaker (using the ON input). The ON input
tion and a self-reset sequence is initiated. may also be used as a command in a “smart switch” ap-
plication.
Electronic Circuit Breaker
Lamp Outage Detector
ON/OFF IN VS
CT CD RD *RSEN 5V TO 44V 3V
0.22µF 0.01µF 100k 0.1Ω 1M
CT DS
Z5U
LTC1153 LAMP
100k
ON/OFF
TO µP STATUS G IRLR024 5k
–
51k 51k 0.5Ω LT1637 OUT
GND SHUTDOWN
5V +
**70°C SENSITIVE
PTC 5V LOAD

OUT = 0V FOR GOOD BULB
3V FOR OPEN BULB
ALL COMPONENTS SHOWN ARE SURFACE MOUNT.
1637 TA05
* IMS026 INTERNATIONAL MANUFACTURING SERVICE, INC. (401) 683-9700
** RL2006-100-70-30-PT1 KEYSTONE CARBON COMPANY (814) 781-1591
LTC1153 • TA01
In this circuit, the lamp is monitored in both the on and
off condition for continuity. In the off condition, the fila-
The LTC1153 is an Electronic Circuit Breaker. Sensed cur- ment pull-down action creates a small test current in the
rent to a load opens the breaker when 100mV is devel- 5kΩ that is detected to indicate a good lamp. If the lamp
oped between the supply input, Vs, and the Drain Sense is open, the 100kΩ pull-up, or the relay contact, provides
pin, DS. To avoid transient, or nuisance trips of the break the op-amp bias current through the 5kΩ, that is oppo-
components RD and CD delay the action for 1msec. A site in polarity. When the lamp is powered and filament
thermistor can also be used to bias the Shutdown input current is flowing, the drop in the 0.05Ω sense resistor
to monitor heat generated in the load and remove power will exceed that of the 5kΩ and a lamp-good detection
should the temperature exceed 70°C in this example. A will still occur. This circuit requires particular Over-the-
feature of the LTC1153 is timed Automatic Reset which Top input characteristics for the op-amp, so part substi-
will try to re-connect the load after 200msec using the tutions are discouraged (however, this same circuit also
0.22µF timer capacitor shown. works properly with an LT1716 comparator, also an Over-
the-Top part).

Fault Sensing-2

47k
–48V 5V
RETURN FUSE
STATUS
R1 R2
100k 100k 3 MOC207
SUPPLY A SUPPLY B
1 4 5V
VA OUT F OK OK 0 0
2 MOC207
OV: OVERVOLTAGE
FUSE A
5V
STATUS = VA = VB 0
= VA ≠ VB 1
≠ VA = VB 1
MOC207 ≠ VA ≠ VB 1*
6
R3
SUPPLY A 1/4W
F2 D2
–48V


Conventional H-Bridge Current Monitor A common monitoring approach in these systems is to
BATTERY BUS
amplify the voltage on a “flying” sense resistor, as
+ shown. Unfortunately, several potentially hazardous fault
ground at a motor terminal. Another complication is the
noise introduced by the PWM activity. While the PWM
noise may be filtered for purposes of the servo law, in-
RS +
DIFF formation useful for protection becomes obscured. The
AMP best solution is to simply provide two circuits that indi-
IM –
offer the protection features needed. In these situations,
DN374 F03 the best solution is the one that derives the load informa-
generally driven by H-bridge MOSFET arrays using pulse-
width-modulation (PWM) methods to vary the com-
manded torque. In these systems, there are two main
purposes for current monitoring. One is to monitor the
current in the load, to track its performance against the
desired command (i.e., closed-loop servo law), and an-
other is for fault detection and protection features.

Fault Sensing-3

Single Supply 2.5V Bidirectional Operation with Fast Current Sense with Alarm
ISENSE
TO RSENSE
CHARGER/
1 8 1µF
FIL– FIL+
–
LT1787
2 VS VS+ 7
2.5V
3 VBIAS 6
DNC
C3
ROUT
4 5 1000pF
VEE
VOUT
–
A1 VOUT A
2.5V +
1M
LT1495 The LT1995 is shown as a simple unity gain difference
5% LT1389-1.25 amplifier. When biased with split supplies the input cur-
1787 F07
The LT1787’s output is buffered by an LT1495 rail-to-rail 100mΩ sense resistor. With 32MHz of bandwidth and
op-amp configured as an I/V converter. This configura- 1000V/usec slew rate the response of this sense ampli-
tion is ideal for monitoring very low voltage supplies. The fier is fast. Adding a simple comparator with a built in
LT1787’s VOUT pin is held equal to the reference voltage reference voltage circuit such as the LT6700-3 can be
appearing at the op amp’s non-inverting input. This al- used to generate an over-current flag. With the 400mV
lows one to monitor supply voltages as low as 2.5V. The reference the flag occurs at 4A.
may drive following circuitry more effectively than the
high output impedance of the LT1787. The I/V converter

Battery Current Monitor
IL
RSENSE
CHARGE
0.1Ω

DISCHARGE 5V 12V

– RA RA
–
A2 A1
1/2 LT1495 RA RA 1/2 LT1495
+
+

2N3904 2N3904
DISCHARGE
OUT
CHARGE
OUT
VO = IL ()
RB
RA
RSENSE

VO
= 1V/A
IL 1495 TA05


Fault Sensing-4


Digitizing
In many systems the analog voltage quantity indicating To see other chapters in this Application Note, return to
current flow must be input to a system controller. In this the Introduction.
chapter several examples of the direct interface of a cur-
rent sense amplifier to an A to D converter are shown.

Sensing Output Current
VCC
0V TO 1V
12V
VCSRC
VCSNK
EN
+IN VCC
V+
ISRC
ISNK RS
TSD 0.2Ω
LT1970 OUT
SENSE+
SENSE– RLOAD
FILTER
–IN V–
VEE
COMMON

LT1787 R4
255k
–12V VS– VS+
RG RF BIAS
–12V 12V
R1
20k
60.4k –
VEE VOUT
R2 LT1880 2.5V
10k + ±5mV/mA

R3 1kHz FULL CURRENT
–12V 20k –12V BANDWIDTH

0V TO 5V A/D

1970 F10

OPTIONAL DIGITAL FEEDBACK

The LT1970 is a 500mA power amplifier with voltage in a microprocessor controlled system. For closed loop
programmable output current limit. Separate DC voltage control of the current to a load an LT1787 can monitor
inputs and an output current sensing resistor control the the output current. The LT1880 op amp provides scaling
maximum sourcing and sinking current values. These and level shifting of the voltage applied to an A-to-D
control voltages could be provided by a D-to-A Converter Converter for a 5mV/mA feedback signal.

Digitizing-1

Split or Single Supply Operation, Bidirectional Output into A/D
1Ω
1%
IS = ±125mA VCC
5V
VSRCE 1 8
FIL– FIL+
≈4.75V LT1787
– VS+ 7 10µF
2 VS
16V
3 VBIAS 6 1
DNC
7
20k CONV
VEE 4 5 VOUT (±1V) 2 6 CLOCKING
VEE AIN LTC1404 CLK
–5V VOUT 3 CIRCUITRY
OPTIONAL SINGLE VREF 5
DOUT
SUPPLY OPERATION: 10µF GND
DISCONNECT VBIAS 16V 4 8
FROM GROUND
AND CONNECT IT TO VREF. 10µF DOUT
REPLACE –5V SUPPLY 16V
WITH GROUND. VEE 1787 TA02
OUTPUT CODE FOR ZERO –5V
CURRENT WILL BE ~2430

In this circuit, split supply operation is used on both the LT1787 pin 6 is driven by VREF, the bidirectional meas-
LT1787 and LT1404 to provide a symmetric bidirectional urement range is slightly asymmetric due to VREF being
measurement. In the single-supply case, where the somewhat greater than mid-span of the ADC input range.

16-Bit Resolution Unidirectional 12-Bit Resolution Unidirectional
Output into LTC2433 ADC Output into LTC1286 ADC
RSENSE
TO I = 100A 0.0016Ω
LOAD
1 8 2.5V TO 60V
FIL– FIL+
–
LT1787HV +
2 VS VS 7 R1 C1
5V
VBIAS 6 15k 1µF
3
DNC
ROUT
20k VREF VCC
4 5 CS
VEE +IN
VOUT LTC1286 CLK TO µP
–IN D
GND OUT
C2 1787 TA01
0.1µF LT1634-1.25
VOUT = VBIAS + (8 • ILOAD • RSENSE)

While the LT1787 is able to provide a bidirectional out-
The LTC2433-1 can accurately digitize signal with source put, in this application the economical LTC1286 is used
impedances up to 5kΩ. This LTC6101 current sense cir- to digitize a unidirectional measurement. The LT1787 has
cuit uses a 4.99kΩ output resistance to meet this re- a nominal gain of eight, providing a 1.25V full-scale out-
quirement, thus no additional buffering is necessary. put at approximately 100A of load current.

Digitizing-2


Current Control
This chapter collects a variety of techniques useful in To see other chapters in this Application Note, return to
generating controlled levels of current in circuits. the Introduction.

800 mA/1A White LED Current Regulator

D2
LED WARNING! VERY BRIGHT
DO NOT OBSERVE DIRECTLY
L1 LED
3µH CURRENT
D1 0.030Ω LT6100
B130 VS+ VS–
VIN
3.3V TO 4.2V VIN VSW VCC
SINGLE Li-Ion
LT3436
22µF – +
SHDN FB 16V
LED 124k CER
ON GND VC 1210 VOUT

VEE A4 A2
4.7µF MMBT2222
6.3V 8.2k 0.1µF OPEN: 1A
CER 4.99k CLOSED: 800mA

6100 TA02
D1: DIODES INC.
D2: LUMILEDS LXML-PW09 WHITE EMITTER
L1: SUMIDA CDRH6D28-3R0

The LT6100 is configured for a gain of either 40V/V or ered. The LT3436 is a boost switching regulator which
50V/V depending on whether the switch between A2 and governs the voltage/current supplied to the LED. The
VEE is closed or not. When the switch is open (LT6100 switch “LED ON” connected to the SHDN pin allows for
gain of 40V/V), 1A is delivered to the LED. When the external control of the ON/OFF state of the LED.
switch is closed (LT6100 gain of 50V/V), 800mA is deliv-

Bidirectional Current Source Two Terminal Current Regulator
+V
3 7
VCTL +
6
LT1990
2
– 4 RSENSE
1
REF –V
ILOAD
ILOAD = VCTL/RSENSE ≤ 5mA
EXAMPLE: FOR RSENSE =100Ω,
OUTPUT IS 1mA PER 100mV INPUT
1990 AI03

The LT1990 is a differential amplifier with integrated pre-
cision resistors. The circuit shown is the classic Howland The LT1635 combines an op amp with a 200mV refer-
current source, implemented by simply adding a sense ence. Scaling this reference voltage to a potential across
resistor. resistor R3 forces a controlled amount of current to flow
from the +terminal to the –terminal. Power is taken from
the loop.

Current Control-1

Variable Current Source Precision Voltage Controlled Current Source

A basic high-side current source is implemented at the
output, while an input translation amplifier section pro-
vides for flexible input scaling. A Rail-to-Rail input capa-
bility is required to have both amplifiers in one package, The ultra-precise LTC2053 instrumentation amplifier is
since the input stage has common-mode near ground configured to servo the voltage drop on sense resistor R
and the second section operates near VCC. to match the command VC. The LTC2053 output capabil-
ity limits this basic configuration to low current applica-
Precision Voltage Controlled Current tions.
Source with Ground Referred Input and Output
5V
Switchable Precision Current Source
INPUT 3 5
0V TO 3.7V
+
1 4V TO 44V
LTC2050 +
4 4.7µF LT1004-1.2
– 2
2k R
0.68µF
R*
5V
+
1k
3
LT1637 TP0610
1/2 LTC6943
7 6
– IOUT = 1.2
R
9 IOUT e.g., 10mA = 120Ω
1µF 1µF 1k
10 SHDN *OPTIONAL FOR LOW OUTPUT CURRENTS, 1637 TA01

R* = R
12 11 VIN
IOUT =

15 14
1000Ω
This is a simple current-source configuration where the
0.001µF
OPERATES FROM A
op amp servos to establish a match between the drop on
SINGLE 5V SUPPLY
6943 • TA01a the sense resistor and that of the 1.2V reference. This
The LTC6943 is used to accurately sample the voltage particular op amp includes a shutdown feature so the
across the 1kΩ sense resistor and translate it to a current source function can be switched off with a logic
ground reference by charge balancing in the 1µF capaci- command. The 2kΩ pull-up resistor assures the output
tors. The LTC2050 integrates the difference between the MOSFET is off when the op amp is in shutdown mode.
sense voltage and the input command voltage to drive
the proper current into load.

Current Control-2

Boosted Bidirectional Controlled Current Source Fast Differential Current Source
+V
15V
R* 2 10pF R* VIN2 – VIN1
7
1k
VIN1 – IOUT =
R
CZT751 6
LT1022
R* 3
3 7 VIN2 +
VCTL + 4
R*
6
LT1990 –15V IOUT RL
2 +
– 4 10µF
RSENSE
1 *MATCH TO 0.01%
REF ILOAD FULL-SCALE POWER BANDWIDTH
= 1MHz FOR IOUTR = 8VP-P
1k = 400kHz FOR IOUTR = 20VP-P
CZT651 MAXIMUM IOUT = 10mAP-P IOUTP-P • RL
COMMON-MODE VOLTAGE AT LT1022 INPUT =
2
–V
LT1022 • TA07
ILOAD = VCTL/RSENSE ≤ 100mA
EXAMPLE: FOR RSENSE =10Ω,
OUTPUT IS 1mA PER 10mV INPUT 1990 AI04
This is a variation on the Howland configuration, where
load current actually passes through a feedback resistor
This is a classical Howland bidirectional current source as an implicit sense resistance. Since the effective sense
implemented with an LT1990 integrated difference ampli- resistance is relatively large, this topology is appropriate
fier. The op amp circuit servos to match the RSENSE for producing small controlled currents.
voltage drop to the input command VCTL. When the load
current exceeds about 0.7mA in either direction, one of 1A Voltage-Controlled Current Sink
the boost transistors will start conducting to provide the
additional commanded current.

0A to 2A Current Source

This is a simple controlled current sink, where the op
amp drives the NMOSFET gate to develop a match be-
tween the 1Ω sense resistor drop and the VIN current
command. Since the common-mode voltage seen by the
The LT1995 amplifies the sense resistor drop by 5V/V op amp is near ground potential, a “single-supply” or
and subtracts that from VIN, providing an error signal to Rail-to-Rail type is required in this application.
an LT1880 integrator. The integrated error drives the
PMOSFET as required to deliver the commanded current.

Current Control-3

Voltage Controlled Current Source Adjustable High-Side Current Source
V+ VCC RSENSE
5V 0.2Ω

1k
2.5k
0.0033µF
LT1004-1.2 –
100Ω Q1
1/2 LT1366
MTP23P06
– RP
RS 10k
+
1Ω ILOAD
+
40k
+IN 5V < VCC < 30V
LTC6101 0A < ILOAD < 1A AT VCC = 5V
0mA < ILOAD < 160mA AT VCC = 30V
FOR VIN = 0V TO 5V,
IOUT = 500mA TO 0mA Q2
2N4340
IOUT = 100mA/V
LT1366 F07

10µF
+ RLOAD The wide-compliance current source shown takes advan-
tage of the LT1366’s ability to measure small signals near
–
0.2V
REF
1k 24k the positive supply rail. The LT1366 adjusts Q1’s gate
LT3021 VIN
voltage to force the voltage across the sense resistor
(RSENSE) to equal the voltage between VDC and the poten-
tiometer’s wiper. A rail-to-rail op amp is needed because
Adding a current sense amplifier in the feedback loop of the voltage across the sense resistor is nearly the same
an adjustable low dropout voltage regulator creates a as VDC. Q2 acts as a constant current sink to minimize
simple voltage controlled current source. The range of error in the reference voltage when the supply voltage
output current sourced by the circuit is set only by the varies. At low input voltage, circuit operation is limited by
current capability of the voltage regulator. The current the Q1 gate drive requirement. At high input voltage, cir-
sense amplifier senses the output current and feeds back cuit operation is limited by the LT1366’s absolute maxi-
a current to the summing junction of the regulator’s error mum ratings.
amplifier. The regulator will then source whatever current
is necessary to maintain the internal reference voltage at
the summing junction. For the circuit shown a 0V to 5V
control input produces 500mA to 0mA of output current.

Current Control-4

Programmable Constant Current Source
6V D45VH10 0.1Ω IOUT
TO 28V 0A TO 1A
0.1µF
470Ω
LT1121CS8-5
8 1
IN OUT
+
SHDN GND 1µF 0.1µF
0.1µF 10k
5 3 18k
0.1µF 1 8 1%
SENSE AVG
SHUTDOWN 2 7
VN2222LM IOUT PROG
3 LT1620MS8 6
2N3904 GND VCC
IPROG
RPROG
22Ω 4 5
–IN +IN

IOUT = (IPROG)(10,000)
RPROG = 40k FOR 1A OUTPUT
LT1620/21 • TA01

The current output can be controlled by a variable resis- shutdown command to the LT1121 powers down the
tor (RPROG) connected from the PROG pin to ground on LT1620 and eliminates the base-drive to the current regu-
the LT1620. The LT1121 is a low-dropout regulator that lation pass transistor, thereby turning off IOUT.
keeps the voltage constant for the LT1620. Applying a

Snap Back Current Limiting
12V

R2 R1 R3
39.2k 54.9k 2.55k

VCSRC
VCSNK
EN 500mA IMAX
VIN +IN VCC
V+
ISRC 50mA ILOW
ISNK RS
1Ω IOUT 0
TSD
LT1970 OUT
SENSE+
– –500mA
SENSE RL
FILTER
– VCC • R2
–IN V
VEE IMAX ≈
(R1 + R2) • 10 • RS
COMMON
VCC • (R2||R3)
ILOW ≈
[R1 + (R2||R3)] • 10 • RS
RG –12V RF
10k 10k
1970 F04

The LT1970 provides current detection and limiting fea- command to a lower level. When the load condition per-
tures built-in. In this circuit, the logic flags that are pro- mits the current to drop below the limiting level, then the
duced in a current-limiting event are connected in a feed- flags clear and full current drive capability is restored
back arrangement that in turn reduces the current limit automatically.

Current Control-5


Precision
Offset voltage and bias current are the primary sources High Side Power Supply Current Sense
of error in current sensing applications. To maintain pre-
cision operation the use of zero-drift amplifier virtually
eliminates the offset error terms.

the Introduction.

Precision High Side Power Supply Current Sense
1.5mΩ
VREGULATOR

2 – 8
OUT
7 100mV/A The low offset error of the LTC6800 allows for unusually
LTC6800
3 + 6 10k
OF LOAD
CURRENT
low sense resistance while retaining accuracy.
5
4
0.1µF
ILOAD LOAD Second Input R Minimizes
Error Due to Input Bias Current
150Ω
V+
6800 TA01

RIN–
This is a low-voltage, ultra-high-precision monitor featur-
ing a Zero-Drift Instrumentation Amplifier (IA) that pro- RSENSE
vides Rail-to-Rail inputs and outputs. Voltage gain is set RIN+
4 3
by the feedback resistors. Accuracy of this circuit is set + –
by the quality of resistors selected by the user, small- LOAD
2 5
signal range is limited by VOL in single-supply operation.
The voltage rating of this part restricts this solution to
applications of <5.5V. This IA is sampled, so the output is
discontinuous with input changes, thus only suited to 1
LTC6101 VOUT
very low frequency measurements. ROUT

6101 F04
RIN+ = RIN– – RSENSE

The second input resistor decreases input error due
caused by the input bias current. For smaller values of
RIN this may not be a significant consideration.

Precision-1


Wide Range
To measure current over a wide range of values requires To see other chapters in this Application Note, return to
gain changing in the current sense amplifier. This allows the Introduction.
the use of a single value of sense resistor. The alternative
approach is to switch values of sense resistor. Both ap-
proaches are viable for wide range current sensing.

CMPZ4697 VLOGIC
(3.3V TO 5V)
10k 7
M1 3
Si4465 +
VIN
4
ILOAD
RSENSE HI –
10m 8 Q1
5 CMPT5551
VOUT
RSENSE LO 40.2k 6
301 100m 301 301 301
4.7k
1.74M
LTC1540
4 3 4 3
2 1 HIGH
2 + – 5 2 + – 5 RANGE
VIN
619k INDICATOR
(ILOAD > 1.2A)

LTC6101 LTC6101
250mV/A

7.5k
VLOGIC

BAT54C
2.5V/A
R5
7.5k
0 ≤ ILOAD ≤ 10A 6101 F03b


Wide Range-1

Adjust Gain Dynamically for Enhanced Range
RSENSE ISENSE
TO LOAD FROM SOURCE
– VS+
LT6100 VS

– +

5V VCC

FIL
VOUT
VEE A2 A4
6100 TA05

2N7002 5V
0V (GAIN = 50)
(GAIN = 10)

Instead of having fixed gains of 10, 12.5, 20, 25, 40, and
50, this circuit allows selecting between two gain set-
tings. An NMOSFET switch is placed between the two
gain-setting terminals (A2, A4) and ground to provide
selection of gain = 10 or gain = 50, depending on the
state of the gate drive. This provides a wider current
measurement range than otherwise possible with just a
single sense resistor.

Wide Range-2

Current sensor

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